Add SparkFun ESP32-C6 Thing Plus Variant (#9178)

* Add SparkFun ESP32-C6 Thing Plus to boards.txt

* Add SparkFun ESP32-C6 Thing Plus variant

variants/sparkfun_esp32c6_thing_plus/pins_arduino.h

+#ifndef Pins_Arduino_h
+#define Pins_Arduino_h
+
+#include <stdint.h>
+#include "soc/soc_caps.h"
+
+#define PIN_NEOPIXEL        23
+// BUILTIN_LED can be used in new Arduino API digitalWrite() like in Blink.ino
+static const uint8_t LED_BUILTIN = SOC_GPIO_PIN_COUNT+PIN_NEOPIXEL;
+#define BUILTIN_LED  LED_BUILTIN // backward compatibility
+#define LED_BUILTIN LED_BUILTIN  // allow testing #ifdef LED_BUILTIN
+// RGB_BUILTIN and RGB_BRIGHTNESS can be used in new Arduino API neopixelWrite()
+#define RGB_BUILTIN LED_BUILTIN
+#define RGB_BRIGHTNESS 64
+
+static const uint8_t TX = 16;
+static const uint8_t RX = 17;
+
+static const uint8_t SDA = 6;
+static const uint8_t SCL = 7;
+
+static const uint8_t SS   = 5;
+static const uint8_t MOSI = 20;
+static const uint8_t MISO = 21;
+static const uint8_t SCK  = 19;
+
+static const uint8_t A0 = 0;
+static const uint8_t A1 = 1;
+static const uint8_t A2 = 2;
+static const uint8_t A3 = 3;
+static const uint8_t A4 = 4;
+static const uint8_t A5 = 5;
+static const uint8_t A6 = 6;
+
+#endif /* Pins_Arduino_h */