ARM: imx: Enable SDP download in SPL on DH i.MX6 DHSOM

ARM: imx: Enable SDP download in SPL on DH i.MX6 DHSOM

Enable SDP protocol support in SPL for DH i.MX6 DHSOM, now that those
components fit into the SPL due to LTO.

To start U-Boot via SDP upload on i.MX6 DHSOM based board, proceed as follows:
- Compile imx_usb [1] .
- Power off the i.MX6 DHSOM based board.
- Connect both USB-serial console and USB-OTG miniB ports to host PC.
- Switch board to USB boot mode.
- Power on the board.
- Verify using '$ dmesg' that a new device has been detected as follows:
    New USB device found, idVendor=15a2, idProduct=0054, bcdDevice= 0.01
    New USB device strings: Mfr=1, Product=2, SerialNumber=0
    Product: SE Blank ARIK
    Manufacturer: Freescale SemiConductor Inc

- Upload U-Boot SPL:
$ imx_usb u-boot-with-spl.imx

- Wait for SPL to come up, the following print ought to be the last on
  UART console:
SDP: handle requests...

- Upload U-Boot proper:
$ imx_usb u-boot.img

[1] https://github.com/boundarydevices/imx_usb_loader.git

Signed-off-by: Marek Vasut <marex@denx.de>