MIPS: ralink: add a bootrom dumper module

This patch adds a trivial driver that allows userland to extract the bootrom of
a SoC via debugfs.

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/8002/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>