boards: arm: qemu_cortex_m0: rework timer driver

We rework the timer driver for Cortex-M0 QEMU Board.
The driver is adapted to follow the (recent) nRF RTC
driver logic, and adaptations are made simply because
the driver is based on the TIMER peripheral instead of
the RTC.

A couple of tweaks are added due to QEMU issues with the
simulation of the TIMER peripheral.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>