boot: zephyr: boards: nrf54h20dk cpuapp add chosen code part

Add the required zephyr,code-partition which is typically included
by the common app.overlay to the nrf54h20dk_nrf54h20_cpuapp board
overlay.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>