usb: host: xhci-rcar: Move R-Car reg definitions

Move xhci-rcar reg definitions to a header file for the preparation of adding
support for RZ/G3E XHCI that has different register definitions.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20250916150255.4231-5-biju.das.jz@bp.renesas.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>