ARM: dts: meson8b: add the I2C_A, PWM_C and UART_B pins

These are used for example on the Endless Mini (EC-100):
- I2C_A is connected to the Realtek RT5640 audio codec
- PWM_C (GPIODV_9) is connected to a PWM regulator which is used for
  VCCK (CPU voltage supply)
- UART_B is connected to the Bluetooth module (of the RTL8723BS SDIO
  wifi and Bluetooth combo chip)

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>