ARM: imx53: clk: add ARM clock

The ARM clock is a virtual clock feeding the ARM partition of
the SoC. It controls multiple other clocks to ensure the right
sequencing when cpufreq changes the CPU clock rate.

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