[POWERPC] Consolidate PowerPC 750 cputable features

The 750 CPU_FTR macros have quite a bit of duplication in them.  Consolidate
them to use CPU_FTRS_750 and only list the unique features for derivatives.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>