pwm-backlight: Add optional enable GPIO

To support a wider variety of backlight setups, introduce an optional
enable GPIO. Legacy users of the platform data already have a means of
supporting GPIOs by using the .init(), .exit() and .notify() hooks. DT
users however cannot use those, so an alternative method is required.

In order to ease the introduction of the optional enable GPIO, make it
available in the platform data first, so that existing users can be
converted. Once that has happened a second patch will add code to make
use of it in the driver.

Signed-off-by: Thierry Reding <treding@nvidia.com>