MIPS: EMMA: Move arch/mips/emma2rh/ into arch/mips/emma/

git mv arch/mips/{emma2rh,emma} and fixups Makefiles.  We'll put all NEC
EMMA series based machines there in the future.

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>