serial: fsl_lpuart: Use common rs485 device tree parsing function

We just got a common helper for parsing the rs485 specific
device tree properties. Use it and drop the open coded parser.

As a side effect this adds support for parsing rs485-rts-delay and
rs485-rx-during-tx. As the driver doesn't support this though, probing
fails if these are defined.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>