ARM: imx: add CPU clock type

This implements a virtual clock used to abstract away
all the steps needed in order to change the ARM clock,
so we don't have to push all this clock handling into
the cpufreq driver.

While it will be used for i.MX53 at first it is generic
enough to be used on i.MX6 later on.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>