powerpc/mp: add support for discontiguous cores

Some SOCs have discontiguously-numbered cores, and so we can't determine the
valid core numbers via the FRR register any more.  We define
CPU_TYPE_ENTRY_MASK to specify a discontiguous core mask, and helper functions
to process the mask and enumerate over the set of valid cores.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>