powerpc/radix: Update MMU cache

With radix there is no MMU cache. Hence we don't need to do anything in
update_mmu_cache().

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>