Move do_fixup* for libfdt into common code

Moved the generic fixup handling code out of cpu/mpc5xxx and cpu/mpc8260
into common/fdt_support.c and renamed:

do_fixup()	-> do_fixup_by_path()
do_fixup_u32() 	-> do_fixup_by_path_u32()

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>