boards: arm: nrf5340pdk: clean up sram0_shared definition

We do not want sram0_shared to represent physical ram;
this is just a portion of sram reserved for shared memory
between Application and Network MCU. Therfore, we remove
the 'mmio' compatible property and transform this node to
a reserved-memory node definition, inside which we define
the sram0_shared node along with its reg property.

In addition we correct the documentation about the shared
memory, stressing that it is placed after the image RAM of
nrf5340 Application MCU (not after the secure SRAM).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>