dt-bindings: arm: stm32: Modify STM32MP15x Phytec board items types

dt-bindings: arm: stm32: Modify STM32MP15x Phytec board items types

As Phytec manages different SoM configurations with different STM32MP15
SoC versions, modify the phyBOARD and SoM compatible items to "enum"
instead of "const".
The description concerns PHYTEC SoM equipped with STM32MP157
("st,stm32mp157" is "const").
Also add comments in front of the enum items to be able to identify the
compatible string with the phyBOARD/phyCORE names.

Signed-off-by: Christophe Parant <c.parant@phytec.fr>
Link: https://lore.kernel.org/r/20251210101611.27008-4-c.parant@phytec.fr
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>