boards : rpi_pico : Add persistent partition for Pico and Pico_w

Add a 16 KB 'storage' partition at the end of the Pico flash.
That size is sufficient to hold the two test files created by
the 'littlefs' sample.

Signed-off-by: Andreas Wolf <awolf002@gmail.com>

Author: Snap-A

boards/raspberrypi/rpi_pico/rpi_pico-common.dtsi

@@ -79,13 +79,24 @@
 
 		/*
 		 * Usable flash. Starts at 0x100, after the bootloader. The partition
-		 * size is 2MB minus the 0x100 bytes taken by the bootloader.
+		 * size is 2MB minus the 0x100 bytes taken by the bootloader, and minus
+		 * the size of the storage partition, set to 16 KB (see below).
 		 */
 		code_partition: partition@100 {
 			label = "code-partition";
-			reg = <0x100 (DT_SIZE_M(2) - 0x100)>;
+			reg = <0x100 (DT_SIZE_M(2) - 0x100 - DT_SIZE_K(16))>;
 			read-only;
 		};
+
+		/*
+		 * Area for placing a file system for persistent storage. Starts at the end
+		 * of the code partition. The size of 16 KB is chosen to have the capacity for
+		 * a minimal 'littlefs' file system with 4 blocks.
+		 */
+		storage_partition: partition@1fc000 {
+			label = "storage-partition";
+			reg = <(DT_SIZE_M(2) - DT_SIZE_K(16)) (DT_SIZE_K(16))>;
+		};
 	};
 };