dt-bindings: imx8mq-clock: Add the missing ARM clock

Add the missing ARM clock which will be used by cpufreq

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
[sboyd@kernel.org: Fixed numbering in dt header]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>