ppc: rename arch/ppc/ to arch/powerpc/ ...

In old days, Linux supported PowerPC with two arch directories,
arch/ppc/ and arch/ppc64/.

Linux commit 564ee7a5668e ("[PATCH] powerpc: Move arch/ppc*/kernel/vecemu.c
to arch/powerpc") started the migration to arch/powerpc/, and
commit 917f0af9e5a9 ("powerpc: Remove arch/ppc and include/asm-ppc")
finished it.

This commit aligns the directory name with the current Linux.

I did 'git mv arch/ppc/ arch/powerpc/', and fixed up some hard-coded
arch/ppc paths.

Barebox has stuck to arch/ppc/ for a long time. To keep the backward
compatibility, I added the following to the top Makefile.

  # Support ARCH=ppc for backward compatibility
  ifeq ($(ARCH),ppc)
         SRCARCH := powerpc
  endif

Both ARCH=ppc and ARCH=powerpc work in the same way.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>