drivers: mbox: add initial driver for nRF VEVIF event

Add a mailbox driver for VEVIF events (VPR irq).
The driver can be built in either 'rx' or 'tx' configuration.
The VPR sends the event, so it uses the 'tx' configuration,
while the master core uses the 'rx' configuration of the driver
to receive the VPR events.

Signed-off-by: Jakub Zymelka <jakub.zymelka@nordicsemi.no>