i2c: img-scb: Add runtime PM

The i2c-img-scb driver already dynamically enables / disables clocks to
save power, but doesn't use the runtime PM framework. Convert the
driver to use runtime PM, so that dynamic clock management will be
disabled when runtime PM is disabled, and so that autosuspend can be
used to avoid unnecessarily disabling and re-enabling clocks repeatedly
during a sequence of transactions.

Signed-off-by: Ed Blake <ed.blake@sondrel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>