shields: Add support for M5Stack-Core2 base shield.

M5Stack-Core2 comes with a base shield that is connected to the M5Stack
extention connector. It features a MPU6886 6-axis motion tracker
and a SPM1423 microphone.

Signed-off-by: Martin Kiepfer <m.kiepfer@teleschirm.org>

boards/shields/m5stack_core2_ext/Kconfig.shield

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+# Copyright (c) 2023 Martin Kiepfer <mrmarteng@teleschirm.org>
+# SPDX-License-Identifier: Apache-2.0
+
+config M5STACK_CORE2_EXT
+	def_bool $(shields_list_contains,m5stack_core2_ext)

boards/shields/m5stack_core2_ext/doc/index.rst

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+.. _m5stack_core2_ext:
+
+M5Stack-Core2 base shield
+####################################
+
+Overview
+********
+
+M5Stack-Core2 comes with a base shield that is connected to the M5Stack
+extension connector. It features a MPU6886 6-axis motion tracker and a SPM1423
+microphone.
+
+.. figure:: img/m5stack_core2_ext.webp
+        :align: center
+        :alt: M5Stack-Core2-EXT
+        :width: 400 px
+
+        M5Stack-Core2-Extension module
+
+Pins Assignments
+================
+
++----------------------+--------------+
+| Shield Connector Pin | Function     |
++======================+==============+
+| 0                    | GND          |
++----------------------+--------------+
+| 11                   | 3.3V         |
++----------------------+--------------+
+| 16                   | I2C - intSDA |
++----------------------+--------------+
+| 17                   | I2C - intSCL |
++----------------------+--------------+
+
+Programming
+***********
+
+Set ``-DSHIELD=m5stack_core2_ext`` when you invoke ``west build``.
+For example:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/drivers/mpu6050
+   :board: m5stack_core2
+   :shield: m5stack_core2_ext
+   :goals: build
+
+References
+**********
+
+.. target-notes::
+
+.. _datasheet:
+   https://docs.m5stack.com/en/core/core2

boards/shields/m5stack_core2_ext/m5stack_core2_ext.overlay

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+/*
+ * Copyright (c) 2023 Martin Kiepfer <mrmarteng@teleschirm.org>
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&m5stack_mbus_i2c0 {
+	status = "okay";
+
+	mbus_mpu6886: mbus_mpu6886@68 {
+		status = "okay";
+		compatible = "invensense,mpu6050";
+		reg = <0x68>;
+	};
+};