xtensa: xtfpga: Try software restart before simulating CPU reset

Rebooting xtensa images loaded with the '-kernel' option in qemu does
not work. When executing a reboot command, the qemu session either hangs
or experiences an endless sequence of error messages.

  Kernel panic - not syncing: Unrecoverable error in exception handler

Reset code jumps to the CPU restart address, but Linux can not recover
from there because code and data in the kernel init sections have been
discarded and overwritten at this point.

XTFPGA platforms have a means to reset the CPU by writing 0xdead into a
specific FPGA IO address. When used in QEMU the kernel image loaded with
the '-kernel' option gets restored to its original state allowing the
machine to boot successfully.

Use that mechanism to attempt a platform reset. If it does not work,
fall back to the existing mechanism.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/platforms/xtfpga/setup.c

@@ -51,8 +51,12 @@ void platform_power_off(void)
 
 void platform_restart(void)
 {
-	/* Flush and reset the mmu, simulate a processor reset, and
-	 * jump to the reset vector. */
+	/* Try software reset first. */
+	WRITE_ONCE(*(u32 *)XTFPGA_SWRST_VADDR, 0xdead);
+
+	/* If software reset did not work, flush and reset the mmu,
+	 * simulate a processor reset, and jump to the reset vector.
+	 */
 	cpu_reset();
 	/* control never gets here */
 }