i2c: designware: Add runtime PM hooks

It is possible that after entering runtime PM suspend the controller
context is lost due the fact that its power is removed. This happens for
example on Asus T100, an Intel Baytrail based tablet/laptop.

In order to get the controller back to functional state, we need to
implement runtime PM hooks which will re-initialize the hardware during
runtime PM resume. We can re-use the existing system suspend hooks as the
steps to resume/suspend the controller are the same.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>