powerpc/mm/64s: Restrict THP to Radix or HPT w/64K pages

Transparent hugepages (THP) are not supported when using the Hash Page
Table (HPT) MMU with 4K pages.

Currently a HPT-only 4K kernel still allows THP to be enabled, which
is misleading.

Add restrictions to the PPC_THP symbol so that if the kernel is
configured with 4K pages and only the HPT MMU (no Radix), then THP is
disabled.

Note that it's still possible to build a combined Radix/HPT kernel with 4K
pages, which does allow THP to be enabled at build time. As such the HPT
code still needs to provide some THP related symbols, to allow the build
to succeed, but those code paths are never run. See the stubs in
arch/powerpc/include/asm/book3s/64/hash-4k.h.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240823032911.1238471-2-mpe@ellerman.id.au