powerpc/microwatt: add POWER9_CPU, clear PPC_64S_HASH_MMU

Microwatt implements a subset of ISA v3.0 (which is equivalent to
the POWER9_CPU option). It is radix-only, so does not require hash
MMU support.

This saves 20kB compressed dtbImage and 56kB vmlinux size.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211201144153.2456614-19-npiggin@gmail.com