powerpc: Consolidate feature fixup macros for 64/32 bit

Currently we have three versions of MAKE_FTR_SECTION_ENTRY(), the macro that
generates a feature section entry.  There is 64bit version, a 32bit version
and version for 32bit code built with a 64bit kernel.

Rather than triplicating (?) the MAKE_FTR_SECTION_ENTRY() logic, we can
move the 64bit/32bit differences into separate macros, and then only have
one version of MAKE_FTR_SECTION_ENTRY().

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>