dts: nrf5340: Use dual compatible property for mbox nodes

This is a follow-up to commit cf6a58d3.

Restore the "nordic,nrf-ipc" compatible property in mbox nodes for both
nRF5340 cores and use it together with the new "nordic,mbox-nrf-ipc"
one. This way either the MBOX or the IPM driver can be used for these
nodes without further modifications. This eliminates the need to use
overlays in quite a few cases, so remove all those no longer needed
ones (which are also a bit confusing now as they refer to no longer
existing ipc@2a000 and ipc@41012000 nodes).

Restore also the ipc node label removed in the commit mentioned above,
as the label is used in validation of base addresses of nRF DT nodes.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>