boards: nrf54h20dk: Add common files for iron board variants

Define `ipc_conf_iron.dtsi` and `memory_map_iron.dtsi`. In the future,
they will be merged with the base `ipc_conf.dtsi` and `memory_map.dtsi`
respectively. For now, they are used to replace a few devicetree nodes
when building for `nrf54h20dk/nrf54h20/*/iron` board targets.

Additional changes are included:

  * The IPC configuration includes new "nordic,ironside-call" nodes.
  * The memory map includes an updated RAM20 layout. Its subregions are
    placed under the `/reserved-memory` node like before.
  * The memory map also includes an updated MRAM layout. Partitions are
    now placed under a plain "fixed-partitions" node. MCUboot-specific
    node labels are applied in `cpuapp.dts`.

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