i2c: designware: Fix runtime PM for I2C slave mode

I2C slave controller must be powered and active all the time when I2C
slave backend is registered in order to let master address and
communicate with us.

Now if the controller is runtime PM capable it will be suspended after
probe and cannot ever respond to the master or generate interrupts.

Fix this by resuming the controller when I2C slave backend is registered
and let it suspend after unregistering.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>