ARM: mx25: dynamically allocate imx-keypad devices

The mxc-keypad device seems to be the result of an early and partial
merge of the keypad driver.  It's unused and there is no corresponding
driver available, so just remove it.

Cc: Alberto Panizzo <maramaopercheseimorto@gmail.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>