arm64: dts: imx8mq: Add RTC support

Add RTC support for i.MX8MQ.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Tested-by: Chris Spencer <christopher.spencer@sea.co.uk>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>