powerpc/pseries: Split retrieval of processor entitlement data into a helper routine

Split the retrieval of processor entitlement data returned in the H_GET_PPP
hcall into its own helper routine.

Signed-off-by: Nathan Fontenot <nfont@austin.ibm.com>
Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>