usb: cdns3: Adds missing __iomem markers

Patch adds missing __iomem markers in core.h file
and makes some changes in drd.c file related with
these markers.

The lack of __iomem has reported by sparse checker
on parsic architecture.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Pawel Laszczak <pawell@cadence.com>
Signed-off-by: Peter Chen <peter.chen@kernel.org>