snippets: Add snippet to use XIAO serial pins for the console

Make it easy to force any XIAO-shaped controller to use the D6/D7 UART
pins for the console. Especially useful for the upcoming XIAO Debugger that
easily forwards XIAO serial over USB.

Signed-off-by: Peter Johanson <peter@peterjohanson.com>

Author: petejohanson

snippets/xiao-serial-console/README.rst

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+.. _snippet-xiao-serial-console:
+
+XIAO UART/Serial Console Snippet (xiao-serial-console)
+######################################################
+
+Overview
+********
+
+This snippet enables console output over the standard XIAO UART pins, setting ``zephyr,console`` to
+``&xiao_serial``.

snippets/xiao-serial-console/snippet.yml

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+name: xiao-serial-console
+append:
+  EXTRA_CONF_FILE: xiao-serial-console.conf
+  EXTRA_DTC_OVERLAY_FILE: xiao-serial-console.overlay

snippets/xiao-serial-console/xiao-serial-console.conf

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+CONFIG_CONSOLE=y
+CONFIG_UART_CONSOLE=y
+CONFIG_SERIAL=y

snippets/xiao-serial-console/xiao-serial-console.overlay

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+/*
+ * Copyright (c) 2025 Pete Johanson <peter@peterjohanson.com>
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&xiao_serial {
+	status = "okay";
+};
+
+/ {
+	chosen {
+		zephyr,console = &xiao_serial;
+		zephyr,shell-uart = &xiao_serial;
+	};
+};