feat(boards): Add Seeed(uino) XIAO interconnect

* Document the Seeed(uino) XIAO interconnect
* Add metadata files for two upstream boards, XIAO and XIAO BLE.
* Add conf and overlay files to properly configure the boards
  for ZMK use.

app/boards/arm/seeeduino_xiao/seeeduino_xiao.zmk.yml

@@ -0,0 +1,9 @@
+file_format: "1"
+id: seeeduino_xiao
+name: Seeeduino XIAO
+type: board
+arch: arm
+outputs:
+  - usb
+url: https://wiki.seeedstudio.com/Seeeduino-XIAO/
+exposes: [seeed_xiao]

app/boards/arm/seeeduino_xiao_ble/seeeduino_xiao_ble.zmk.yml

@@ -0,0 +1,10 @@
+file_format: "1"
+id: seeeduino_xiao_ble
+name: Seeeduino XIAO BLE
+type: board
+arch: arm
+outputs:
+  - usb
+  - ble
+url: https://wiki.seeedstudio.com/XIAO_BLE/
+exposes: [seeed_xiao]

app/boards/interconnects/seeed_xiao/seeed_xiao.zmk.yml

@@ -0,0 +1,10 @@
+file_format: "1"
+id: seeed_xiao
+name: Seeed XIAO
+type: interconnect
+url: https://wiki.seeedstudio.com/Seeeduino-XIAO/
+manufacturer: Seeed
+description: |
+  The Seeed(uino) XIAO is a popular smaller format micro-controller, that has gained popularity as an alterative
+  to the SparkFun Pro Micro. Since its creation, several pin compatible controllers, such
+  as the Seeeduino XIAO BLE, Adafruit QT Py and Adafruit QT Py RP2040, have become available.

app/boards/seeeduino_xiao.conf

@@ -0,0 +1,5 @@
+CONFIG_CONSOLE=n
+CONFIG_SERIAL=n
+CONFIG_UART_CONSOLE=n
+CONFIG_UART_INTERRUPT_DRIVEN=n
+CONFIG_ZMK_USB=y

app/boards/seeeduino_xiao.overlay

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2022 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+/ {
+	chosen {
+		zephyr,console = &cdc_acm_uart;
+	};
+};
+
+&usb0 {
+	cdc_acm_uart: cdc_acm_uart {
+		compatible = "zephyr,cdc-acm-uart";
+		label = "CDC_ACM_0";
+	};
+};
+

app/boards/seeeduino_xiao_ble.conf

@@ -0,0 +1,8 @@
+
+CONFIG_CONSOLE=n
+CONFIG_SERIAL=n
+CONFIG_UART_CONSOLE=n
+CONFIG_UART_INTERRUPT_DRIVEN=n
+CONFIG_ZMK_USB=y
+CONFIG_ZMK_BLE=y
+

app/boards/seeeduino_xiao_ble.overlay

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2021 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+
+/ {
+	chosen {
+		zephyr,console = &cdc_acm_uart;
+	};
+
+	vbatt {
+		compatible = "zmk,battery-voltage-divider";
+		label = "BATTERY";
+		io-channels = <&adc 7>;
+		power-gpios = <&gpio0 14 (GPIO_OPEN_DRAIN | GPIO_ACTIVE_LOW)>;
+		output-ohms = <1000000>;
+		full-ohms = <(1000000 + 510000)>;
+	};
+};
+
+&adc {
+	status = "okay";
+};
+
+&usbd {
+	cdc_acm_uart: cdc_acm_uart {
+		compatible = "zephyr,cdc-acm-uart";
+		label = "CDC_ACM_0";
+	};
+};
+