i2c-designware: add minimal support for runtime PM

In order to save power the device should be put to low power states
whenever it is not being used. We implement this by enabling minimal
runtime PM support.

There isn't much to do for the device itself as it is disabled once the
last transfer is completed but subsystem/domain runtime PM hooks can save
more power by power gating the device etc.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>