Enable DFU protocol support (via the "dfu" command) on Tegra boards where
USB device/gadget mode is enabled.

Note that for DFU to operate correctly on Tegra, we still need some DFU
fixes/enhancements that are going through the DFU -> USB trees. However,
the code builds just fine without those changes, and applying this patch
now will allow both sets of patches to meet in the main U-Boot tree much
more quickly.

In order to run test/dfu/dfu_gadget_test.sh, you would need to add the
following to the board configuration:

CONFIG_EXT4_WRITE
CONFIG_CMD_EXT4_WRITE

However, I haven't enabled those here, since I believe the main use-case
for DFU on Tegra is raw flash writing, rather than filesystem access, so
we don't need the additional code-size hit. However, I could be persuaded
otherwise! We should probably add a separate test script for raw flash
access.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>