powerpc: Fix early setup to make early_ioremap() work

The immediate problem is that after commit
0bd3f9e953 ("powerpc/legacy_serial: Use early_ioremap()") the kernel
silently reboots on some systems.

The reason is that early_ioremap() returns broken addresses as it uses
slot_virt[] array which initialized with offsets from FIXADDR_TOP ==
IOREMAP_END+FIXADDR_SIZE == KERN_IO_END - FIXADDR_SIZ + FIXADDR_SIZE ==
__kernel_io_end which is 0 when early_ioremap_setup() is called.
__kernel_io_end is initialized little bit later in early_init_mmu().

This fixes the initialization by swapping early_ioremap_setup() and
early_init_mmu().

Fixes: 265c3491c4 ("powerpc: Add support for GENERIC_EARLY_IOREMAP")
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Drop unrelated cleanup & cleanup change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210520032919.358935-1-aik@ozlabs.ru

arch/powerpc/kernel/setup_64.c

@@ -369,11 +369,11 @@ void __init early_setup(unsigned long dt_ptr)
 	apply_feature_fixups();
 	setup_feature_keys();
 
-	early_ioremap_setup();
-
 	/* Initialize the hash table or TLB handling */
 	early_init_mmu();
 
+	early_ioremap_setup();
+
 	/*
 	 * After firmware and early platform setup code has set things up,
 	 * we note the SPR values for configurable control/performance