serial: mxc: Add iMX6SX compatible string

Add compatible string for iMX6SX.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Silvio Fricke <silvio.fricke@softing.com>
Cc: Stefano Babic <sbabic@denx.de>