MIPS: microMIPS: Support dynamic ASID sizing.

Changes for pure microMIPS cores to dynamically determine the ASID
size at boot time.

Includes bug fix https://patchwork.linux-mips.org/patch/5230/



Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>
Signed-off-by: Jonas Gorski <jogo@openwrt.org>