powerpc/mm/64s: Move THP reqs into a separate symbol

Move the Kconfig symbols related to transparent hugepages (THP) under a
separate config symbol, separate from CONFIG_PPC_BOOK3S_64.

The new symbol is automatically enabled if CONFIG_PPC_BOOK3S_64 is
enabled, so there is no behaviour change, except for the existence of
the new PPC_THP symbol.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240823032911.1238471-1-mpe@ellerman.id.au