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

The DFU usage procedure is identical to STM32MP1 DHCOR SoM, see commit
3919aa17 ("ARM: dts: stm32: Add DFU support for DHCOR recovery") ,
except for switching the SoM into DFU mode. By default, the DHCOM SoM
has no dedicated mechanism for setting BOOTn straps into UART/USB mode,
therefore to enter DFU mode, the SoC must fail to boot from boot media
which can be selected by the BOOTn strap override mechanism first and
then fall back to DFU mode.

In case of a SoM with pre-populated BOOTn strap override button, power
the system off, remove microSD card (if applicable), hold down the BOOTn
strap override button located between eMMC and SoM edge connector, power
on the SoM. The SoC will fail to boot from SD card and fall back into
DFU mode.

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>