ppc: Split pcr_mask settings into supported bits and the register mask

The current pcr_mask values are ambiguous: Should these be the mask
that defines valid bits in the PCR register? Or should these rather
indicate which compatibility levels are possible? Anyway, POWER6 and
POWER7 should certainly not use the same values here. So let's
introduce an additional variable "pcr_supported" here which is
used to indicate the valid compatibility levels, and use pcr_mask
to signal the valid bits in the PCR register.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
