ARM: dts: imx: modify the clocks used by cpufreq driver

As on i.MX7D, we using a virtual arm clk for CPU frequency
scaling, so correct the clocks info used by the cpufreq driver.

Signed-off-by: Bai Ping <b51503@freescale.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>