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 <robh@kernel.org>
Reviewed-by: Jun Li <jun.li@nxp.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>