i2c: Runtime PM for SuperH Mobile I2C

This patch modifies the SuperH Mobile I2C driver to support
Runtime PM. These changes is all that is needed for proper
Runtime PM support in this driver. Driver callbacks for
Runtime PM are empty because the device registers are always
re-initialized after pm_runtime_get_sync().

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