Enable IO20 on ESP32

Some newer ESP32 variants (like ESP32-PICO-V3 and ESP32-PICO-MINI-02)
do implement this pin and it can be used as a normal GPIO.

Fixes #6016
Fixes #6837

Closes https://github.com/espressif/esp-idf/pull/6918

(cherry picked from commit 6deaefde69c3f068d39b76d628b05b21fd142b79)

components/soc/esp32/gpio_periph.c

@@ -35,7 +35,7 @@ const uint32_t GPIO_PIN_MUX_REG[GPIO_PIN_COUNT] = {
     IO_MUX_GPIO17_REG,
     IO_MUX_GPIO18_REG,
     IO_MUX_GPIO19_REG,
-    0,
+    IO_MUX_GPIO20_REG, // This corresponding pin is only available on ESP32-PICO-V3 chip package
     IO_MUX_GPIO21_REG,
     IO_MUX_GPIO22_REG,
     IO_MUX_GPIO23_REG,