ARM: mxs: Fix UARTs on M28EVK

Make use of RTS/CTS lines on AUART0 and register AUART1 and AUART2
which are routed onto a pin header.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>