sh: rework clocksource and sched_clock

Rework and simplify the sched_clock and clocksource code. Instead
of registering the clocksource in a shared file we move it into the
tmu driver. Also, add code to handle sched_clock in the case of no
clocksource.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>