zephyr/boards: Add nrf5340dk board configuration.

Add support for the nrf5340dk.  This DK has a MX25R64 8mb external QSPI
flash chip.

Compile using:

    $ west build -b nrf5340dk/nrf5340/cpuapp

Signed-off-by: Patrick Joy <[email protected]>

Author: ThinkTransit

ports/zephyr/boards/nrf5340dk_nrf5340_cpuapp.conf

@@ -0,0 +1,2 @@
+CONFIG_FLASH=y
+CONFIG_FLASH_MAP=y

ports/zephyr/boards/nrf5340dk_nrf5340_cpuapp.overlay

@@ -0,0 +1,16 @@
+// Replace default internal storage partition with external flash
+
+/delete-node/ &storage_partition;
+
+&mx25r64 {
+	partitions {
+		compatible = "fixed-partitions";
+		#address-cells = <1>;
+		#size-cells = <1>;
+
+		storage_partition: partition@0 {
+			reg = <0x00000000 0x800000>;
+			label = "storage";
+		};
+	};
+};