doc: dt-binding: ci-hdrc-usb2: add property for samsung picophy

Add two parameters which are used to tune USB signal for samsung picophy,
which is used at imx7d, imx8mm, and imx8mn.

Reviewed-by: Rob Herring <[e-mail]>
Reviewed-by: Jun Li <[e-mail]>
Signed-off-by: Peter Chen <[e-mail]>

Documentation/devicetree/bindings/usb/ci-hdrc-usb2.txt

···
   It's recommended to specify the over current polarity.
 - power-active-high: power signal polarity is active high
 - external-vbus-divider: enables off-chip resistor divider for Vbus
+- samsung,picophy-pre-emp-curr-control: HS Transmitter Pre-Emphasis Current
+  Control. This signal controls the amount of current sourced to the
+  USB_OTG*_DP and USB_OTG*_DN pins after a J-to-K or K-to-J transition.
+  The range is from 0x0 to 0x3, the default value is 0x1.
+  Details can refer to TXPREEMPAMPTUNE0 bits of USBNC_n_PHY_CFG1.
+- samsung,picophy-dc-vol-level-adjust: HS DC Voltage Level Adjustment.
+  Adjust the high-speed transmitter DC level voltage.
+  The range is from 0x0 to 0xf, the default value is 0x3.
+  Details can refer to TXVREFTUNE0 bits of USBNC_n_PHY_CFG1.
 
 Example: