dt-bindings: arm: stm32: narrow DH STM32MP1 SoM boards

The bindings for DH STM32MP1 SoM boards allows invalid combinations,
e.g. st,stm32mp153 SoC on a dh,stm32mp157c-dhcom-som SoM.

Split the enums to properly match valid setups.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>