Documentation: bindings: add dt doc for Rockchip USB Type-C PHY

This patch adds a binding that describes the Rockchip USB Type-C PHY
for rk3399

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>