phy: rcar-gen3-usb2: remove HSUSB registers handling

Since the related driver (CPG/MSSR driver) only manages the first module
clock, this driver should not handle the HSUSB registers. So, this patch
removes the HSUSB registers handling.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>