powerpc/mm: Replace open coded CONTEXT_BITS value

To clarify the meaning for future readers, replace the open coded
19 with CONTEXT_BITS

Reviewed-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>