ARM: mach-mxs/mx28evk: Only register devices if their GPIO requests succeeded

Currently framebuffer and MMC devices are registered even if their associated
GPIO pins fail to be requested.

Change the logic so that the registration of such devices only occurs if their
GPIO requests succeeded.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>