arch/tile: provide kernel support for the tilegx USB shim

This change adds support for accessing the USB shim from within the
kernel.  Note that this change by itself does not allow the kernel
to act as a host or as a device; it merely exposes the built-in on-chip
hardware to the kernel.

The <arch/usb_host.h> and <arch/usb_host_def.h> headers are empty at
the moment because the kernel does not require any types or definitions
specific to the tilegx USB shim; the generic USB core code is all we need.
The headers are left in as stubs so that we don't need to modify the
hypervisor header (drv_usb_host_intf.h) from upstream.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>