target/ppc: more use of the PPC_*() macros

Also introduce utilities to manipulate bitmasks (originaly from OPAL)
which be will be used in the model of the XIVE interrupt controller.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>