powerpc/mm: Convert 4k insert from asm to C

This is similar to 64K insert. May be we want to consolidate

Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>