drivers: lpuart: enable rs485 mode

NXP LPUART IP supports rs485 mode when transceiver driver enable
using RTS. Allow setting rs485 mode up via the "nxp,rs485-mode"
dts property. "nxp,rs485-de-active-low" dts property can be used
for set RTS polarity.

Signed-off-by: Grixa Yrev <GrixaYrev@yandex.ru>