gpio: pca954x: Add vcc regulator and enable it

Some i2c gpio devices are connected to a switchable power supply
which needs to be enabled prior to probing the device. This patch
allows the drive to enable the devices vcc regulator prior to probing.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>