usb: dwc3: core: split host address space

This fix prevents a problem with dwc3 and host mode where
we were requesting the entire memory region in dwc3/core.c,
thus preventing xhci-plat from ever ioremapping its own address space.

Signed-off-by: Ido Shayevitz <idos@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>