usb: dwc2: move debugfs code to a separate file

Prepare to add more debug code. Moreover, don't save dentry * for
each file in struct dwc2_hsotg as clean up is done with
debugfs_remove_recursive(). s3c_hsotg_delete_debug() is removed
altogether for the same reason.

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>