riscv: TRANSPARENT_HUGEPAGE: depends on MMU

Fix a Kconfig warning and many build errors:

WARNING: unmet direct dependencies detected for COMPACTION
  Depends on [n]: MMU [=n]
  Selected by [y]:
  - TRANSPARENT_HUGEPAGE [=y] && HAVE_ARCH_TRANSPARENT_HUGEPAGE [=y]

and the subseqent thousands of build errors and warnings.

Fixes: e88b3331 ("riscv: mm: add THP support on 64-bit")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>