drivers: mbox: add initial driver for nRF VEVIF

Add a mailbox driver for VEVIF. The driver can be built in either
'local' or 'remote' configuration. This depends on the existence of the
'interrupts' property, which signals that the instance is managed
locally. VEVIF is, as expected, always managed by a VPR core.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>