imx6: wandboard: fix dwc_ahsata build errors when DM enabled

imx6: wandboard: fix dwc_ahsata build errors when DM enabled

Enable CONFIG_AHCI to fix errors:
drivers/ata/dwc_ahsata.c: In function `ahci_init_one':
drivers/ata/dwc_ahsata.c:868:21: error: `struct blk_desc' has no member named `priv'
sata_dev_desc[pdev].priv = uc_priv;
     ^
drivers/ata/dwc_ahsata.c: In function `init_sata':
drivers/ata/dwc_ahsata.c:891:30: error: `struct blk_desc' has no member named `priv'
uc_priv = sata_dev_desc[dev].priv;
...

Also enable DM_SCSI to fix migration build warning.

Signed-off-by: Anatolij Gustschin <agust@denx.de>