media: adv7183: Convert to GPIO descriptors

This driver is using two GPIO numbers passed as platform data.

No board file in the kernel defines this however, so we can
just change the mechanism without side effects.

Let's just switch it to use GPIO descriptors and add some
comments on how to provide these.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>