samples: Added fade sample

Added a fade sample to demonstrate how to use analogWrite API

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>

Author: soburi

samples/fade/CMakeLists.txt

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+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+
+set(DTC_OVERLAY_FILE $ENV{ZEPHYR_BASE}/../modules/lib/Arduino-Zephyr-API/variants/${BOARD}/${BOARD}.overlay)
+
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(fade)
+
+target_sources(app PRIVATE src/app.cpp)
+
+zephyr_compile_options(-Wno-unused-variable -Wno-comment)

samples/fade/README.rst

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+.. _fade:
+
+Fade
+####
+
+Overview
+********
+
+The Fade sample gradually increases/decreases the voltage of the output pin.
+When connecting the LED to the output pin, the LED blinks gradually.
+
+Building and Running
+********************
+
+Build and flash Fade sample as follows,
+
+```sh
+$> west build  -p -b arduino_nano_33_ble samples/basic/fade/ -DZEPHYR_EXTRA_MODULES=/home/$USER/zephyrproject/modules/lib/Arduino-Core-Zephyr
+
+$> west flash --bossac=/home/$USER/.arduino15/packages/arduino/tools/bossac/1.9.1-arduino2/bossac
+```

samples/fade/prj.conf

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+CONFIG_CPLUSPLUS=y
+CONFIG_ARDUINO_API=y
+CONFIG_GPIO=y
+CONFIG_PWM=y

samples/fade/src/app.cpp

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+/*
+ * Copyright (c) 2022 TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <Arduino.h>
+#include "zephyrSerial.h"
+
+const int led = 3;  // PWM output pin.
+const int increments = 5;
+const int wait_ms = 10;
+
+void setup() {
+  /* Pin that use as the PWM output must not configure by pinMode() */
+}
+
+void loop() {
+  int value = 0;
+  while (value < 256) {
+    analogWrite(led, value);
+    value += increments;
+    delay(wait_ms);
+  }
+
+  value = 255;
+  while (value >= 0) {
+    analogWrite(led, value);
+    value -= increments;
+    delay(wait_ms);
+  }
+}