ARM: imx: enable cpufreq device on i.mx7d

Add a cpufreq-dt device on i.MX7D, using the
generic cpufreq-dt for CPU frequency scaling.

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