ARM: mxs: Add mx28evk_auart_console target

The target uses AUART 3 instead of the DUART for console output.

Signed-off-by: Andreas Wass <andreas.wass@dalelven.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Marek Vasut <marex@denx.de>