target/ppc: Use class fields to simplify LPCR masking

target/ppc: Use class fields to simplify LPCR masking

When we store the Logical Partitioning Control Register (LPCR) we have a
big switch statement to work out which are valid bits for the cpu model
we're emulating.

As well as being ugly, this isn't really conceptually correct, since it is
based on the mmu_model variable, whereas the LPCR isn't (only) about the
MMU, so mmu_model is basically just acting as a proxy for the cpu model.

Handle this in a simpler way, by adding a suitable lpcr_mask to the QOM
class.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>