SCMI-enabled boards may restrict access to resources like clocks, resets
and regulators to the secure world.

A normal world bootloader or kernel compatible with the non-SCMI-enabled
board is thus not guaranteed to be able to deal with the SCMI variant.

It follows, that the SCMI-enabled board is not compatible with the
non-SCMI enabled board, so drop that compatible.

This change is motivated by the barebox' bootloader's use of bootloader
specification files[1][2]: barebox for non-SCMI DK2 will compare its
own top-level "stm32mp157c-dk2" compatible with all compatibles
listed in the device tree referenced by each bootloader spec file.
If the boot medium contains a configuration with
compatible = "st,stm32mp157c-dk2-scmi", "st,stm32mp157c-dk2", "st,stm32mp157";
it will match, because of the second compatible and boot a kernel with
SCMI enabled, although no SCMI may exist on the platform.

[1]: https://uapi-group.org/specifications/specs/boot_loader_specification/
[2]: https://www.barebox.org/doc/latest/user/booting-linux.html#boot-loader-specification

Fixes: 8e14ebb1 ("dt-bindings: arm: stm32: Add SCMI version of STM32 boards (DK1/DK2/ED1/EV1)")
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>