clk: imx: Add a virtual arm clk on i.mx7d

Add a virtual arm clk to abstract the actual steps
when changing the ARM core frequency.So we can using
the 'cpufreq-dt' driver on i.MX7D/Solo.

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