backlight: lm3630a: Add an enable gpio for the HWEN pin

For now just enable it in the probe function to allow I2C
access. Disabling also means resetting the register values
to default and according to the datasheet does not give
power savings.

Tested on Kobo Clara HD.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Reviewed-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>