drivers: counter: gecko: Add SYSRTC stimer support

SiLabs' sleeptimer driver supports several hardware peripherals, of
which the counter driver so far only supports the RTCC-based variant.

This patch adds support for the SYSRTC-based sleeptimer implementation,
which is required for Gecko SoCs that do not have an RTCC module.

Signed-off-by: Markus Fuchs <markus.fuchs@ch.sauter-bc.com>