powerpc: Remove CONFIG_POWER4_ONLY

Remove CONFIG_POWER4_ONLY, the option is badly named and only does two
things:

- It wraps the MMU segment table code. With feature fixups there is
  little downside to compiling this in.

- It uses the newer mtocrf instruction in various assembly functions.
  Instead of making this a compile option just do it at runtime via
  a feature fixup.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>