remove usage of CONFIG_ARCH_MX1. It's mostly unused anyway, replace
it with cpu_is_mx1() where necessary. Also, depend on
IMX_HAVE_PLATFORM_IMX_FB instead of the architectures directly.

LAKML-Reference: 20110303141244.GQ29521@pengutronix.de
Acked-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>