ARM: imx6: do not use cpu_is_xxx() in clock driver

As we're about to move clock drivers out of arch/arm/mach-imx,
cpu_is_xxx() shouldn't be used any more.  Let's avoid the call by
looking at the device tree machine compatible string to determine
which SoC the clock driver is running on.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>