MIPS: add clkdev.h

For clock device lookup tables to work on MIPS, we need to provide this
architecture specific header file.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3700/


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