Some useful macros can be used to specified the **direct** GPIO (UART module connected to pads through direct IO mux without the GPIO mux) number of a UART channel, or vice versa.
The pin name can be omitted if specify the channel of a GPIO num.
e.g.
1.``UART_NUM_2_TXD_DIRECT_GPIO_NUM`` is the GPIO number of UART channel 2 TXD pin (17);
2.``UART_GPIO19_DIRECT_CHANNEL`` is the UART channel number of GPIO 19 (channel 0);
3.``UART_CTS_GPIO19_DIRECT_CHANNEL`` is the UART channel number of GPIO 19, and GPIO 19 must be a CTS pin (channel 0).
