clocksource: SuperH MTU2 Timer driver

This patch adds a MTU2 driver for the SuperH architecture.

The MTU2 driver is a platform driver with early platform
support to allow using a MTU2 channel as only clockevent
during system bootup.

Clocksource on sh2a is currently unsupported due to code
generation issues with 64-bit math, so at this point only
periodic clockevent support is in place.

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