mmc_da: implement d_dump method, sddadump

sddadump has been derived from sddastart.

mmc_sim interface has grown a new method, cam_poll, to support polled
operation.

mmc_sim code has been changed to provide a sim_poll hook only if the
controller implements the new method. The hooks is implemented in terms
of the new mmc_sim_cam_poll method.
Additionally, in-progress CCB-s now have CAM_REQ_INPROG status to
satisfy xpt_pollwait().

mmc_sim_cam_poll method has been implemented in dwmmc host controller.

Reviewed by: manu, mav, imp
MFC after: 2 weeks
Relnotes: perhaps
Differential Revision: https://reviews.freebsd.org/D33843