Use software emulated RS485 direction control to provide RS485 API

Currently it is not possible to use rs485 as pointer to
rs485_config struct in struct uart_port is NULL in case we
configure the port through device tree.
Signed-off-by: NHeiko Schocher <hs@denx.de>

Link: https://lore.kernel.org/r/20190913050105.1132080-1-hs@denx.deSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>