boards: arm: nrf5340pdk: define sram0_image for image(s) RAM

We should not be using sram0 for image SRAM in nrf5340pdk.
sram0 represents the physical SRAM and that one includes the
shared memory between the two M33 CPUs on the SoC. We should
not be re-sizing sram0 to account for the shared RAM; instead
we would like to have sram0 representing the whole available
SRAM.

For that, we define a new memory node, sram0_image to
represent the 'image' SRAM that is available for Zephyr
on the board. sram0_image is the chosen image SRAM for
default builds, i.e. when TrustZone is ignored
(TRUSTED_EXECUTION_SECURE is not defined).

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