zephyr/boards/rpi_pico: Add board configuration for rpi_pico.

Although the rpi_pico can already build and run with the zephyr port, this
configuration improves it in a number of ways:
- Use the USB CDC ACM as the REPL, rather than just a UART.
- Enable I2C and SPI, and add I2C1.
- Enable a filesystem, which matches exactly the rp2 port's RPI_PICO
  configuration.  So switching between zephyr and rp2 is possible and will
  retain the filesystem.
- Make the MicroPython GC heap make the most use of the available RAM.

Signed-off-by: Damien George <damien@micropython.org>

ports/zephyr/boards/rpi_pico.conf

@@ -0,0 +1,19 @@
+# Disable floating point hardware.
+CONFIG_FPU=n
+
+# Configure serial console over USB CDC ACM.
+CONFIG_USB_DEVICE_STACK_NEXT=y
+CONFIG_USBD_CDC_ACM_CLASS=y
+CONFIG_UART_LINE_CTRL=y
+
+# Disable networking.
+CONFIG_NETWORKING=n
+
+# Hardware features.
+CONFIG_FLASH=y
+CONFIG_FLASH_MAP=y
+CONFIG_I2C=y
+CONFIG_SPI=y
+
+# MicroPython config.
+CONFIG_MICROPY_HEAP_SIZE=196608

ports/zephyr/boards/rpi_pico.overlay

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2025 Damien P. George
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/ {
+	chosen {
+		/* Use USB CDC ACM as the console. */
+		zephyr,console = &cdc_acm_uart0;
+	};
+};
+
+/* Delete defined partitions and make a layout matching the rp2 port RPI_PICO configuration. */
+/delete-node/ &code_partition;
+&flash0 {
+	partitions {
+		compatible = "fixed-partitions";
+		#address-cells = <1>;
+		#size-cells = <1>;
+
+		/* Code slot: 640 KiB - 0x100 placed after second-stage bootloader. */
+		code_partition: partition@100 {
+			label = "code-partition";
+			reg = <0x00000100 (DT_SIZE_K(640) - 0x100)>;
+			read-only;
+		};
+
+		/* Storage slot: 1408 KiB placed after code_partition. */
+		storage_partition: partition@a0000 {
+			label = "storage";
+			reg = <0x000a0000 DT_SIZE_K(1408)>;
+		};
+	};
+};
+
+&zephyr_udc0 {
+	cdc_acm_uart0: cdc_acm_uart0 {
+		compatible = "zephyr,cdc-acm-uart";
+	};
+};
+
+&i2c1 {
+	clock-frequency = <I2C_BITRATE_STANDARD>;
+	status = "okay";
+	pinctrl-0 = <&i2c1_default>;
+	pinctrl-names = "default";
+};