clocksource/imx: Define clocksource for mx27

The rework of the imx clocksource driver missed to add an entry for
imx27 which results in a boot failure on those machines.

Add the proper CLOCKSOURCE_OF_DECLARE() entry for imx27 and map it to
the imx21 init.

Fixes: bef11c88

 'ARM: imx: initialize gpt device type for DT boot'
Signed-off-by: Philippe Reynes <tremyfr@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: daniel.lezcano@linaro.org
Cc: fabio.estevam@freescale.com
Cc: shawnguo@kernel.org
Cc: kernel@pengutronix.de
Link: http://lkml.kernel.org/r/1435439504-406-1-git-send-email-tremyfr@gmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>