dt-bindings: phy: qcom,snps-eusb2-repeater: Document qcom,tune-res-fsdif

dt-bindings: phy: qcom,snps-eusb2-repeater: Document qcom,tune-res-fsdif

Document the FS Differential TX Output Resistance Tuning value found on
the eUSB2 repeater on Qualcomm PMICs. The tuning values have special
meanings, being different offsets of the resistance to the default value
in Ohms but the exact meaning is not relevant here, as the correct
tuning is determined by hardware engineers to make sure the electrical
properties are as expected.

Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250709-sm7635-eusb-repeater-v2-1-b6eff075c097@fairphone.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>