powerpc/64s: Make mmu_hash_ops __ro_after_init

The mmu_hash_ops are only assigned to during boot, so mark them
__ro_after_init to prevent any further modification.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240821080745.872151-1-mpe@ellerman.id.au