usb: isp1760: Reorganize header files

The isp1760-rhcd.h header contains PTD constants and structures only
useful for the HCD implementation. It also contains register definitions
that will be needed by common code when implementing support for the
ISP1761 device controller, but doesn't contain the isp1760_hcd structure
definition that will also be used by common code.

Move definitions to the right location and create an isp1760-regs.h to
store register definitions. No change other than moving definitions and
modifying indentation is performed.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>