kontron-sl-mx8mm: Select the CONFIG_CMD_UNZIP option

Select the CMD_UNZIP option so that the 'gzwrite' command
can be used to flash wic.gz image into the eMMC.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>