powerpc/mm: Fix makefile for KASAN

In commit 17312f25 ("powerpc/mm: Move book3s32 specifics in
subdirectory mm/book3s64"), ppc_mmu_32.c was moved and renamed.

This patch fixes Makefiles to disable KASAN instrumentation on
the new name and location.

Fixes: f072015c

 ("powerpc: disable KASAN instrumentation on early/critical files.")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>