Since commit 037385062ca8 ("powerpc/mm/32s: use generic mmu_mapin_ram()
for all blocks.") it is possible to map all blocks as RAM on any PPC32.
Remove related restrictions.
And remove call to wii_memory_fixups() which doesn't do anything else
than checks since commit 5c428d4ec822 ("powerpc/wii: remove
wii_mmu_mapin_mem2()")