dt-bindings: phy: Add STM32MP25 COMBOPHY bindings

dt-bindings: phy: Add STM32MP25 COMBOPHY bindings

Document the bindings for STM32 COMBOPHY interface, used to support
the PCIe and USB3 stm32mp25 drivers.
Following entries can be used to tune caracterisation parameters
 - st,output-micro-ohms and st,output-vswing-microvolt bindings entries
to tune the impedance and voltage swing using discrete simulation results
 - st,rx-equalizer register to set the internal rx equalizer filter value.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Christian Bruel <christian.bruel@foss.st.com>
Link: https://lore.kernel.org/r/20240930170847.948779-2-christian.bruel@foss.st.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>