boards: nordic: nrf54h20dk: refactor RAM memory map

Refactor the default RAM memory map on nrf54h20dk:

Removes use of "nordic,owned-memory" which is no longer needed on
nrf54h20. Reserved memory nodes that were under "nordic,owned-memory"
have been moved directly under reserved-memory.

The memory shared between cpuapp-cpusec and cpurad-cpusec in RAM0x
is no longer used with IronSide, since IPC buffers toward the secure
domain are at new fixed locations. The cpuapp_data region
has been expanded to fill the available space in RAM0x when removing
these shared memory regions.

Signed-off-by: Jonathan Nilsen <jonathan.nilsen@nordicsemi.no>