drivers: tty: serial: omap-serial: add support RS-485/422 mode.

(cherry picked from commit 28c2592e839b0add7f419e06dbd633ff740eac04)
plum-MA: dts: add RS-485 TxEnable/RxEnable resources on ttyO1.

(cherry picked from commit e11a84dd448ea947cb335e826da04b9458788073)