mmc: mx6qsabrelite: fsl_esdhc: Define maximum bus width supported by SabreLite board

Maximum bus width supported by SabreLite board is not 8bit like
all other mx6q specific boards. In case where both host controller
and card support 8bit transfers, they agree to communicate on 8bit
interface while boards like the SabreLite support only 4bit interface.
Due to this reason the mmc 8bit default mode fails on the SabreLite.
To rectify this, define maximum bus width supported by this board (4bit).
If max_bus_width is not defined, it is 0 by default and 8bit width support
will be enabled in host capabilities otherwise host capabilities are modified
accordingly.

It is tested with a MMCplus card.

Signed-off-by: Abbas Raza <Abbas_Raza@mentor.com>
cc: stefano Babic <sbabic@denx.de>
cc: Andy Fleming <afleming@gmail.com>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Andrew Gabbasov <andrew_gabbasov@mentor.com>