riscv: kexec: Fix W=1 build warnings

Fixes the following W=1 build warning(s):

In file included from include/linux/kexec.h:28,
                 from arch/riscv/kernel/machine_kexec.c:7:
arch/riscv/include/asm/kexec.h:45:1: warning: ‘extern’ is not at beginning of declaration [-Wold-style-declaration]
   45 | const extern unsigned char riscv_kexec_relocate[];
      | ^~~~~
arch/riscv/include/asm/kexec.h:46:1: warning: ‘extern’ is not at beginning of declaration [-Wold-style-declaration]
   46 | const extern unsigned int riscv_kexec_relocate_size;
      | ^~~~~
arch/riscv/kernel/machine_kexec.c:125:6: warning: no previous prototype for ‘machine_shutdown’ [-Wmissing-prototypes]
  125 | void machine_shutdown(void)
      |      ^~~~~~~~~~~~~~~~
arch/riscv/kernel/machine_kexec.c:147:1: warning: no previous prototype for ‘machine_crash_shutdown’ [-Wmissing-prototypes]
  147 | machine_crash_shutdown(struct pt_regs *regs)
      | ^~~~~~~~~~~~~~~~~~~~~~
arch/riscv/kernel/machine_kexec.c:23: warning: Function parameter or member 'image' not described in 'kexec_image_info'
arch/riscv/kernel/machine_kexec.c:53: warning: Function parameter or member 'image' not described in 'machine_kexec_prepare'
arch/riscv/kernel/machine_kexec.c:114: warning: Function parameter or member 'image' not described in 'machine_kexec_cleanup'
arch/riscv/kernel/machine_kexec.c:148: warning: Function parameter or member 'regs' not described in 'machine_crash_shutdown'
arch/riscv/kernel/machine_kexec.c:167: warning: Function parameter or member 'image' not described in 'machine_kexec'

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/kexec.h

@@ -42,8 +42,8 @@ struct kimage_arch {
 	unsigned long fdt_addr;
 };
 
-const extern unsigned char riscv_kexec_relocate[];
-const extern unsigned int riscv_kexec_relocate_size;
+extern const unsigned char riscv_kexec_relocate[];
+extern const unsigned int riscv_kexec_relocate_size;
 
 typedef void (*riscv_kexec_method)(unsigned long first_ind_entry,
 				   unsigned long jump_addr,

arch/riscv/kernel/machine_kexec.c

@@ -14,8 +14,9 @@
 #include <asm/set_memory.h>	/* For set_memory_x() */
 #include <linux/compiler.h>	/* For unreachable() */
 #include <linux/cpu.h>		/* For cpu_down() */
+#include <linux/reboot.h>
 
-/**
+/*
  * kexec_image_info - Print received image details
  */
 static void
@@ -39,7 +40,7 @@ kexec_image_info(const struct kimage *image)
 	}
 }
 
-/**
+/*
  * machine_kexec_prepare - Initialize kexec
  *
  * This function is called from do_kexec_load, when the user has
@@ -100,7 +101,7 @@ machine_kexec_prepare(struct kimage *image)
 }
 
 
-/**
+/*
  * machine_kexec_cleanup - Cleanup any leftovers from
  *			   machine_kexec_prepare
  *
@@ -135,7 +136,7 @@ void machine_shutdown(void)
 #endif
 }
 
-/**
+/*
  * machine_crash_shutdown - Prepare to kexec after a kernel crash
  *
  * This function is called by crash_kexec just before machine_kexec
@@ -151,7 +152,7 @@ machine_crash_shutdown(struct pt_regs *regs)
 	pr_info("Starting crashdump kernel...\n");
 }
 
-/**
+/*
  * machine_kexec - Jump to the loaded kimage
  *
  * This function is called by kernel_kexec which is called by the