drivers: nrf_rtc_timer: force init of interrupt and event enabling

Make sure that the interrupt and event routing enable bits have the
reset values in case the Zephyr firmware image is chain-loaded.
When started by MCUboot, these registers values aren't the reset values,
but then Zephyr initializes them in same way. Yet, when the Zephyr
firmware image is chain-loaded by nRF5 bootloader, they may have values
that cause the scheduler to behave in unexpected ways.

Signed-off-by: Giancarlo Stasi <giancarlo.stasi.co@gmail.com>