MIPS: BMIPS: change compile time checks to runtime checks

Allow building for all bmips cpus at the same time by changing ifdefs
to checks for the cpu type, or adding appropriate checks to the
assembly.

Since BMIPS43XX and BMIPS5000 require different IPI implementations,
split the SMP ops into one for each, so the runtime overhead is only
at registration time for them.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6241/