dts: arm: nordic: nrf5340: clean up ipc in dts

Cleaned up the IPC configuration for nRF5340 SoC in Device Tree. This
change fixes the (simple_bus_reg) warning about the missing or empty
reg/ranges property.

This is a follow-up to commit cf6a58d3.

Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>