sh: CMT clockevent platform driver

SuperH CMT clockevent driver.

Both 16-bit and 32-bit CMT versions are supported, but only 32-bit
is tested. This driver contains support for both clockevents and
clocksources, but no unregistration is supported at this point.

Works fine as clock source and/or event in periodic or oneshot mode.
Tested on sh7722 and sh7723, but should work with any cpu/architecture.

This version is lacking clocksource and early platform driver support
for now - this to minimize the amount of dependencies.

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