ARM: dts: stm32: Add DFU support for DHCOR recovery

This patch configures U-Boot SPL for DHCOR SoM to permit DFU upload of
SPL and subsequent u-boot.itb for recovery or commissioning purposes.

To start U-Boot on DHCOR based board, e.g. Avenger96, proceed as follows:
- Install dfu-util on the host PC (in debian this is package 'dfu-util')

- Power off the Avenger96 board.
- Connect both USB-serial console and USB-OTG microB ports to host PC.
- Switch Avenger96 to USB boot mode -- BOOT0..2 switches all set to 0.
- Power on the Avenger96 board.
- Verify using '$ dmesg' that a new device has been detected as follows:
    New USB device found, idVendor=0483, idProduct=df11, bcdDevice= 2.00
    New USB device strings: Mfr=1, Product=2, SerialNumber=3
    Product: DFU in HS Mode @Device ID /0x500, @Revision ID /0x0000
    Manufacturer: STMicroelectronics

- Upload U-Boot SPL:
$ dfu-util -a 1 -D u-boot-spl.stm32
- Upload U-Boot proper:
$ dfu-util -a 0 -D u-boot.itb

- At this point, SPL will wait for user to press "Ctrl-C" on serial
  console. When ready to interact with U-Boot, press Ctrl-C to start
  the bootloader.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>