Implement the bs_sr_<N> generic functions based on the generic
mips implementation calling the generic bs_w_<N> functions in a loop.

ral(4) (rt2860.c) panics in RAL_SET_REGION_4() because bs_sr_4()
is NULL.  It seems ral(4) and ti(4) might be the only consumers of
these functions I could find quickly so keeping them in C rather than asm.