MIPS: PMC-Sierra: Move board Makefile parts to their own Platform file

Signed-off-by: Shane McDonald <mcdonald.shane@gmail.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1354/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>