ARM: dts: mxs: Add AUART2 pinmux

Add 2-pin pinmux settings for AUART2.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>