usb: dwc3: add Keystone specific glue layer

Add Keystone platform specific glue layer to support
USB3 Host mode.

[ balbi@ti.com : fix order of clk_disable() and
	platform_device_unregister() ]

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: WingMan Kwok <w-kwok2@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>