ARM: mach-imx: Support for DryIce RTC in i.MX53

This patch enables support for i.MX53 in addition to i.MX25 by providing a
dummy clock on i.MX53 since this one doesn't have a separate clock for internal
RTC but the driver requests one.

Signed-off-by: Roland Stigge <stigge@antcom.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>