ARM: imx: new helper function imx_add_platform_device

This should be a globally available function, see

	http://thread.gmane.org/gmane.linux.kernel/998881/focus=998882



Until this hits mainline create a similar function available for imx
platforms only.

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