powerpc: make memremap_compat_align 64s-only

memremap_compat_align is only relevant when ZONE_DEVICE is selected.
ZONE_DEVICE depends on ARCH_HAS_PTE_DEVMAP, which is only selected
by PPC_BOOK3S_64.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211201144153.2456614-13-npiggin@gmail.com