rs485 DT properties

// 8250-omap:
// XXX rs485 properties ignored by 8250 in kernel 4.19 ... has this been fixed yet?
rts-gpios = GPIO;       // optional, requires kernel 5.3 or later
rs485-rts-active-low;   // optional (boolean).  contrary to the name, this makes rts active-high.

// omap-serial:
rts-gpio = GPIO;        // required
rs485-rts-active-high;  // optional (boolean).  contrary to the name, this makes rts active-low

// common:
rs485-rts-delay = <DELAY_BEFORE_SEND DELAY_AFTER_SEND>;  // optional
rs485-rx-during-tx;                     // optional (boolean)
linux,rs485-enabled-at-boot-time;       // optional (boolean)