powerpc/slb: Define an enum for the bolted indexes

This patch defines macros for the three bolted SLB indexes we use.
Switch the functions that take the indexes as an argument to use the
enum.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>