defconfig: dra7xx_evm: enable CONFIG_BLK and disk driver model for SCSI

Enable disk driver model for dra7xx_evm as dwc_ahci supports
driver model. As a consequence we must also enable CONFIG_BLK and
CONFIG_DM_USB.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Dropped CONFIG_SPL_PHY=y in sandbox_spl to fix build error:
Signed-off-by: Simon Glass <sjg@chromium.org>