target-ppc: Introduce Decoder Macros for DFP

Add decoder macros for the various Decimal Floating Point
instruction forms.  Illegal instruction masks are used to not only
guard against reserved instruction field use, but also to catch
illegal quad word forms that use odd-numbered floating point registers.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
