configs: stm32mp25: Enable OF_UPSTREAM_BUILD_VENDOR
Initially, only one STM32MP25 based board was available, the
stm32mp257f-ev1 board which was set by default in stm32mp25_defconfig.
Since commit
79f3e77133bd ("Subtree merge tag 'v6.16-dts' of dts repo [1] into dts/upstream")
we inherited of a second MP25 based board which is the stm32mp257f-dk board.
Enable OF_UPSTREAM_BUILD_VENDOR and set OF_UPSTREAM_VENDOR to allow all
STMicroelectronics DT compilation.
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>