arm64: dts: meson: enable rtc node on Khadas VIM1/VIM2 boards

Enable the rtc node on VIM1/VIM2 boards so users can simply attach a power
cell and use the on-board RTC without modifying the device-tree.

Cold boot with no cell attached is gracefully handled:

VIM2:~ # dmesg | grep rtc
[    7.716150] rtc-hym8563 1-0051: no valid clock/calendar values available
[    7.716957] rtc-hym8563 1-0051: registered as rtc0
[    7.729850] rtc-hym8563 1-0051: no valid clock/calendar values available
[    7.729877] rtc-hym8563 1-0051: hctosys: unable to read the hardware clock
[    8.126768] rtc-hym8563 1-0051: no valid clock/calendar values available

Warm boot (and any boot with cell attached) recalls stored values resulting
in consistently faster (re)boot times:

VIM2:~ # dmesg | grep rtc
[    7.441671] rtc-hym8563 1-0051: registered as rtc0
[    7.442663] rtc-hym8563 1-0051: setting system clock to 2020-11-16T05:49:59 UTC (1605505799)

Suggested-by: Artem Lapkin <art@khadas.com>
Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20201116064147.12062-1-christianshewitt@gmail.com