[POWERPC] Fix PowerPC 750CL and 750GX CPU features

PowerPC 750CL has high BATs.  The patch below adds a CPU_FTRS_750CL that
includes that.  Without it, the original firmware mappings in the high BATs
aren't cleared which continue to override the linux translations.

It also adds CPU_FTR_COMMON to CPU_FTRS_750GX for completeness.

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