clk: imx8mq: Add the missing ARM clock

Add the ARM clock as imx_clk_cpu type.
Will be used by cpufreq.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>