imx6ulevk: Configure machine to use u-boot SPL

imx6ulevk: Configure machine to use u-boot SPL

At first, this machine was supported only by u-boot-imx U-Boot provider, however
as soon as u-boot-fslc included this support, the U-Boot provider was changed.

During the change of default U-Boot provider, the SPL configuration end up being
missed.

The needed SPL configuration is set up in this patch.

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>