dts: nordic: nrf54h20_enga: add PPR VEVIF nodes

Add a new nodes for PPR's VEVIF. In app cores, VEVIF registers are part
of the VPR peripheral, so it is exposed as a child node (since it
requires its own properties, eg #mbox-cells). In VPR, it's a CPU child
since it's not a memory-mapped peripheral, but used with CSRs.

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