spapr: Small cleanup of PPC MMU enums

The PPC MMU types are sometimes treated as if they were a bit field
and sometime as if they were an enum which causes maintenance
problems: flipping bits in the MMU type (which is done on both the 1TB
segment and 64K segment bits) currently produces new MMU type
values that are not handled in every "switch" on it, sometimes causing
an abort().

This patch provides some macros that can be used to filter out the
"bit field-like" bits so that the remainder of the value can be
switched on, like an enum. This allows removal of all of the
"degraded" types from the list and should ease maintenance.

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>