dts: arm: silabs: Add rtcc and sysrtc bindings

Different Series 2 devices have different RTC IPs, despite sharing
a HAL driver. Introduce separate bindings for the different IPs, and
use a chosen node to select the node to use for timekeeping.

A chosen node was selected over a nodelabel since chosen nodes can
be overridden by board-level dts and devicetree overlays, while
nodelabels can't.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>