ARM: remove support for mxc91231

Since support for mxc91231 was introduced 2009 it only saw patches that
were part of (mxc or arm) global cleanups. The only supported machine
only had 4 devices (2x UART, sdhc, watchdog).

Cc: Dmitriy Taychenachev <dimichxp@gmail.com>
LAKML-Reference: 1302211482-17926-1-git-send-email-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>