boards: silabs: siwx91x: Add support for PSRAM

Radio Board 4342A has 8MB of PSRAM connected to the Quad-SPI memory
controller.

The memory controller of the 4342A is preconfigured by the bootloader.
So, the PSRAM work out of the box without any manual configuration.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>

Author: jerome-pouiller

boards/silabs/radio_boards/siwx917_rb4342a/siwx917_rb4342a.dts

@@ -31,6 +31,13 @@
 		dht0 = &si7021;
 	};
 
+	psram: psram@a000000 {
+		compatible = "zephyr,memory-region", "mmio-sram";
+		reg = <0xa000000 DT_SIZE_M(8)>;
+		zephyr,memory-region = "psram";
+		zephyr,memory-attr = <( DT_MEM_ARM(ATTR_MPU_RAM) )>;
+	};
+
 	leds {
 		compatible = "gpio-leds";