dts: nrf5340_cpuapp: remove incorrect sram node

The sram1 node in nrf5340_cpuapp.dtsi represents the SRAM accessible
to the network core on the SoC.

However, while the network core can access the app core's SRAM, the
app core cannot access the net core's SRAM. Therefore, the sram1 node
should only appear in DTSI files for the net core, and not the app
core.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>