ARM: imx: convert to new leds-gpio registration helper

This gets rid of per machine struct platform_device definitions and allows
to move the platform data and led definition to .init.rodata.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>