configs: e850-96: Enable USB gadget and fastboot

configs: e850-96: Enable USB gadget and fastboot

Now that USB phy and dwc3 glue layer was added for Exynos850, USB gadget
is functional on E850-96 board. Enable next features to make it useful:

  - Exynos850 USB PHY driver (needed for all USB functions)
  - dwc3 generic driver
  - USB gadget
  - CONFIG_DM_USB_GADGET: needed for DWC3 glue layer to instantiate the
    peripheral driver, i.e. dwc3_generic_peripheral_probe()
  - USB VID and PID
  - DFU
  - Fastboot (including flashing to eMMC boot partitions)

As all Exynos firmware binaries (including U-Boot) are contained in eMMC
boot partition A (mmc0boot0), because that's where Boot ROM code jumps,
it might be useful to be able to flash that area with fastboot. Other
more fine grained choices for updating the firmware would be DFU and EFI
Capsule Update mechanism.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>