ppc: Enable full relocation to RAM

The following changes allow U-Boot to fully relocate from flash to
RAM:
 - Remove linker scripts' .fixup sections from the .text section
 - Add -mrelocatable to PLATFORM_RELFLAGS for all boards
 - Define CONFIG_RELOC_FIXUP_WORKS for all boards

Previously, U-Boot would partially relocate, but statically initialized
pointers needed to be manually relocated.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>