boards: shields: npm1100_ek: add initial support

Add initial support for the nPM1100 EK. The EK is expected to be
connected to Arduino header pins (D2/3/4/5).

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

boards/shields/npm1100_ek/Kconfig.defconfig

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+# Copyright (c) 2023 Nordic Semiconductor ASA
+# SPDX-License-Identifier: Apache-2.0
+
+if SHIELD_NPM1100_EK
+
+config REGULATOR
+	default y
+
+endif # SHIELD_NPM1100_EK

boards/shields/npm1100_ek/Kconfig.shield

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+# Copyright (c) 2023 Nordic Semiconductor ASA
+# SPDX-License-Identifier: Apache-2.0
+
+config SHIELD_NPM1100_EK
+	def_bool $(shields_list_contains,npm1100_ek)

boards/shields/npm1100_ek/doc/index.rst

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+.. _npm1100_ek:
+
+nPM1100 EK
+##########
+
+Overview
+********
+
+The nPM1100 EK lets you test different functions and features of the nPM1100
+Power Management Integrated Circuit (PMIC).
+
+.. figure:: npm1100_ek.jpg
+   :alt: nPM1100 EK
+   :align: center
+
+   nPM1100 EK
+
+Requirements
+************
+
+The nPM1100 EK board is not designed to fit straight into an Arduino connector.
+However, the Zephyr shield is designed expecting it to be connected to the
+Arduino shield pins. This allows to use the shield with any host board that
+supports the Arduino connector. The connections are:
+
++-----------+--------------+
+| PMIC Pin  | Arduino Pin  |
++===========+==============+
+| ISET      | D2           |
+| MODE      | D3           |
++-----------+--------------+
+
+Usage
+*****
+
+The shield can be used in any application by setting ``SHIELD`` to
+``npm1100_ek``.
+
+References
+**********
+
+- `nPM1100 EK Manual <https://infocenter.nordicsemi.com/topic/ug_npm1100_ek/UG/nPM1100_EK/intro.html>`_

boards/shields/npm1100_ek/npm1100_ek.overlay

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+/*
+ * Copyright (C) 2023 Nordic Semiconductor ASA
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/ {
+	npm1100_ek_pmic: pmic {
+		compatible = "nordic,npm1100";
+
+		nordic,iset-gpios = <&arduino_header 8 GPIO_ACTIVE_HIGH>; /* D2 */
+
+		npm1100_ek_buck: BUCK {
+			nordic,mode-gpios = <&arduino_header 9 GPIO_ACTIVE_HIGH>; /* D3 */
+		};
+	};
+};