ARM: dts: mxs: Add SSP2 SD mux

Add pinmux configuration for SSP2 port in SD mode, both for
the 4-bit and 8-bit case.

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