rockchip: xhci: remove DTS parsing for PHY (which is unused)

The xhci wrapper-driver for Rockchip searches the DTS to find its
child node compatbile with 'rockchip,rk3399-usb3-phy' to retrieve the
base-address of the PHY.  However, this is currently broken (and
always has been), returning NULL.  However, the (wrongly) retrieved
base-address is never used.

We thus remove this code for now.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>