ARM: dts: stm32: Switch to using upstream DT on DH STM32 DHSOM

ARM: dts: stm32: Switch to using upstream DT on DH STM32 DHSOM

Enable OF_UPSTREAM to use upstream DT and add st/ prefix to the
DEFAULT_DEVICE_TREE. And thereby directly build DTB from dts/upstream/src/
including *-u-boot.dtsi from arch/$(ARCH)/dts/ directory.

The previous setup used generic SoC prefix like stm32mp15xx-dhco* for
generic DTs which could be used on any STM32MP15xx DHSOM variant. The
new setup uses specific SoC prefix stm32mp157c-dhco* to match Linux DT
names. Since the hardware present on STM32MP153 and STM32MP157 is not
enabled in the board configuration and not supported by U-Boot except
for the DSI host, using the existing Linux DTs poses no issue even on
plain STM32MP151A based SoMs.

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