ARM: dts: i.MX51: Separate TXD/RXD and RTS/CTS pinmux entries for UARTs

RTS/CTS pins can be used for different purposes, so create separate
definitions for these pins.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>