ARM: dts: stm32: phycore-stm32mp15: Rename device tree files

ARM: dts: stm32: phycore-stm32mp15: Rename device tree files

Rename "stm32mp157c-phycore-*" device tree for the following reasons:

- The name of the dts should match to the phyBOARD name and not the name
of the SoM ("phycore-stm32mp1-3" was initialy coming from the name of
the yocto machine from meta-phytec).
- PHYTEC manages different SoM configurations with different STM32MP15x
SoC versions, so common dtsi files starting with "stm32mp15xx-*" should
be used (as it is done for ST boards for example).
- Add "-rdk" as suffix (for "Rapid Development Kit") to match our others
phytec boards dts names (imx6, imx6ul,..).
- "model" property is updated to introduce the name "phyBOARD-Sargas".

Signed-off-by: Christophe Parant <c.parant@phytec.fr>
Link: https://lore.kernel.org/r/20251210101611.27008-2-c.parant@phytec.fr
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>