This patch set uart_rts_gpios as an optional gpio pin.
Some boards (e.g. px3se-sdk) do not use the rts flow control,
and the correspoding uart-rts pin is occupied as some other
functions(e.g. irq pin).
Change-Id: Ia402dc2b74c305d83d3e1ccbedc1c913b270c828
Signed-off-by: Shunqian Zheng <zhengsq@rock-chips.com>