boards: nordic: nrf54h20dk: make RAM3x DMA region larger for cpurad

Some tests are failing on nrf54h20 cpurad in non-obvious manner
because of this memory region being too small.
Instead of adding overlays to each individual application,
make this region larger at expense of cpuapp equivalent.

Signed-off-by: Nikodem Kastelik <nikodem.kastelik@nordicsemi.no>