powerpc: Consolidate CPU and firmware feature fixup macros

The CPU and firmware feature fixup macros are currently spread across
three files, firmware.h, cputable.h and asm-compat.h.  Consolidate them
into their own file, feature-fixups.h

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>