clocksource: SuperH TMU Timer driver

This patch adds a TMU driver for the SuperH architecture.

The TMU driver is a platform driver with early platform
support to allow using a TMU channel as clockevent or
clocksource during system bootup or later.

Clocksource or clockevent can be selected.
Both periodic and oneshot clockevents are supported.

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