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

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

[ Upstream commit 808cf33d4817c730008de9b2736b357708a3d7f6 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>