MFD: ucb1x00-core: add missing ucb1x00_enable()/ucb1x00_disable()

ucb1x00_enable() and ucb1x00_disable() are used for power saving on the
SIB interface, allowing the host supplied clock to be disabled when not
required.  We require drivers which access the ucb1x00 to ensure that
they have enabled the clock prior to accessing the device, and they
should disable it once they're done.

As we don't expect gpiolib users to be aware of this detail, we must
make these calls in the gpiolib interfaces.  Add them.

Also add them to the resume method, which needs to re-establish the
GPIO pin settings.

Acked-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>