mx6sabresd: Add Serial Download Protocol support

Add Serial Download Protocol support (SDP), which allows loading
SPL and u-boot.img via imx_usb_loader tool as explained in
doc/README.sdp.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>

configs/mx6sabresd_defconfig

@@ -18,6 +18,9 @@ CONFIG_SPL=y
 CONFIG_SPL_EXT_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_OS_BOOT=y
+CONFIG_SPL_USB_HOST_SUPPORT=y
+CONFIG_SPL_USB_GADGET_SUPPORT=y
+CONFIG_SPL_USB_SDP_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set
@@ -31,6 +34,7 @@ CONFIG_CMD_PART=y
 CONFIG_CMD_PCI=y
 CONFIG_CMD_SF=y
 CONFIG_CMD_USB=y
+CONFIG_CMD_USB_SDP=y
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y