[nrf noup] boards: Add nordic,pm-ext-flash for nrf5340dk

Adding nordic,pm-ext-flash chosen pointing to mx25r64 as external flash
for partition manager.

Signed-off-by: Dominik Ermel <[email protected]>

Author: de-nordic

boards/nordic/nrf5340dk/nrf5340dk_nrf5340_cpuapp.dts

@@ -18,5 +18,6 @@
 		zephyr,code-partition = &slot0_partition;
 		zephyr,sram-secure-partition = &sram0_s;
 		zephyr,sram-non-secure-partition = &sram0_ns;
+		nordic,pm-ext-flash = &mx25r64;
 	};
 };