usb: host: modify description for MTK xHCI config

Due to all MediaTek SoCs with xHCI host controller use this
driver, remove limitation for specific SoCs

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>