bugfix(uart): workaround for uart fifo reset issue

bugfix(uart): workaround for uart fifo reset issue

Reported from github:
https://github.com/espressif/esp-idf/issues/1219
https://github.com/espressif/esp-idf/issues/1202

After providing a simple code to digital team, they confirmed that this is a hardware bug.

Root cause:
The fifo reset signal is incorrectly connected
If we want to reset tx fifo of UART2, we have to set txfifo_rst bit of both UART1 and UART2
If we want to reset rx fifo of UART2, we have to set rxfifo_rst bit of both UART1 and UART2

Workaround:
we don't use fifo rst bit in driver.

Documentation:
Digital team would update TRM and give more explanation about this issue.