usb: make the MTK XHCI driver compile for older MIPS SoCs

The MIPS based MT7621 shares the same XHCI core as the newer generation of
ARM based SoCs. The driver works out of the box and we only need to make it
buildable in Kconfig.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>