powerpc: Make CMO paging space pool ID and page size available

During platform setup, save off the primary/secondary paging space
pool IDs and the page size.  Added accessors in hvcall.h for these
variables.  This is needed for a subsequent fix.

Submitted-by: Robert Jennings <rcj@linux.vnet.ibm.com>

Signed-off-by: Paul Mackerras <paulus@samba.org>