spl: dfu: add dfu support in SPL

Traditionally the DFU support is available only
as part 2nd stage boot loader(u-boot) and DFU
is not supported in SPL.

The SPL-DFU feature is useful for boards which
does not have MMC/SD, ethernet boot mechanism
to boot the board and only has USB inteface.

This patch add DFU support in SPL with RAM
memory device support to load and execute u-boot.
And then leverage full functionality DFU in
u-boot to flash boot inital binary images to
factory or bare-metal boards to memory devices
like SPI, eMMC, MMC/SD card using USB interface.

This SPL-DFU support can be enabled through
Menuconfig->Boot Images->Enable SPL-DFU support

Signed-off-by: Ravi Babu <ravibabu@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>