Simplify board specific code by specifying the power_control
gpio direct in platform data.
Move registration of the GPIO to the driver so we no longer
need to duplicate this for each board.

As an intended side-effect there is no longer
any references to platform_data outside atmel_lcdc_register()
so remove it from struct atmel_lcdfb_info

The implementation assumes that GPIO=0 is the same as no power control.
This prevents us from using any GPIO=0 for power control,
but this is not considered a problem for current users.
Future DT users will not have this limitation.

This commit include a fix so we will actually power
down if requested. Previously this was hardcoded to ON.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>