imx: nitrogen6x: Make use of both uSD and SD slots to load script or kernel on Sabrel...

imx: nitrogen6x: Make use of both uSD and SD slots to load script or kernel on Sabrelite board

Sabrelite board has two solts: 0 is SD3 (bottom) slot and 1 is uSD4 (top) slot.
This patch makes use of both slots instead of only one.

Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Eric Nelson <eric.nelson@boundarydevices.com>
Reviewed-by: Eric Nelson <eric.nelson@boundarydevices.com>
Acked-by: Eric Nelson <eric.nelson@boundarydevices.com>
Reviewed-by: Eric Nelson <eric.nelson@boundarydevices.com>
Acked-by: Eric Nelson <eric.nelson@boundarydevices.com>