rpi_pico2: rp2350: Add PIO support and extend samples

Add initial support for the RP2350's PIO peripherals, extend the
existing example under samples/boards/raspberrypi/rpi_pico/uart_pio to
demonstrate this on the Raspberry Pi Pico 2, and update the board's
documentation.

Signed-off-by: Andrew Featherstone <andrew.featherstone@gmail.com>
Signed-off-by: Manuel Aebischer <manuel.aebischer@belden.com>

Author: ajf58

boards/raspberrypi/rpi_pico2/doc/index.rst

@@ -69,6 +69,9 @@ hardware features:
    * - UART
      - :kconfig:option:`CONFIG_SERIAL`
      - :dtcompatible:`raspberrypi,pico-uart`
+   * - UART (PIO)
+     - :kconfig:option:`CONFIG_SERIAL`
+     - :dtcompatible:`raspberrypi,pico-uart-pio`
 
 Connections and IOs
 ===================

dts/arm/raspberrypi/rpi_pico/rp2350.dtsi

@@ -386,6 +386,29 @@
 			status = "disabled";
 		};
 
+		pio0: pio@50200000 {
+			compatible = "raspberrypi,pico-pio";
+			reg = <0x50200000 DT_SIZE_K(4)>;
+			clocks = <&clocks RPI_PICO_CLKID_CLK_SYS>;
+			resets = <&reset RPI_PICO_RESETS_RESET_PIO0>;
+			status = "disabled";
+		};
+
+		pio1: pio@50300000 {
+			compatible = "raspberrypi,pico-pio";
+			reg = <0x50300000 DT_SIZE_K(4)>;
+			clocks = <&clocks RPI_PICO_CLKID_CLK_SYS>;
+			resets = <&reset RPI_PICO_RESETS_RESET_PIO1>;
+			status = "disabled";
+		};
+
+		pio2: pio@50400000 {
+			compatible = "raspberrypi,pico-pio";
+			reg = <0x50400000 DT_SIZE_K(4)>;
+			clocks = <&clocks RPI_PICO_CLKID_CLK_SYS>;
+			resets = <&reset RPI_PICO_RESETS_RESET_PIO2>;
+			status = "disabled";
+		};
 	};
 
 	pinctrl: pin-controller {

samples/boards/raspberrypi/rpi_pico/uart_pio/boards/rpi_pico2_rp2350a_m33.overlay

@@ -0,0 +1,8 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Copyright (c) 2024 Andrew Featherstone <andrew.featherstone@gmail.com>
+ */
+
+/* Pico 2 is compatible with the Pico 1, so reuse. */
+#include "rpi_pico.overlay"