sched: Move all scheduler bits into kernel/sched/

(based on
https://github.com/CyanogenMod/android_kernel_motorola_msm8960dt-common/commit/391e43da797a96aeb65410281891f6d0b0e9611c
)
There's too many sched*.[ch] files in kernel/, give them their own
directory.

(No code changed, other than Makefile glue added.)