watchdog: shwdt: Basic clock framework support.

This plugs in basic clock framework support for the watchdog. As it's an
optional MSTP bit, we don't particularly care if a platform has provided
it or not, though a valid clock will need to be available for the more
complex overflow period calculations found on newer parts -- this will be
addressed later.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>