boards: nordic: Flatten shared_ramxx_region nodes

Update the default memory maps for `nrf54h20dk` and `nrf9280pdk` to
remove the `shared_ram20_region` and `shared_ram3x_region` nodes,
because their child nodes no longer need to be grouped together:

  * IPC buffers in RAM20 are statically allocated.
  * DMA buffers in RAM3x have separate access owners.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>