ARM: dts: rockchip: Add rtc node for VMARC SOM

Add the hym8563 rtc found on the rk3288 variant of the VMARC SOM.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Link: https://lore.kernel.org/r/20201023181814.220974-2-jagan@amarulasolutions.com


[split out of the original patch, as it was a change unrelated
 to the commit description]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>