Introduce sim_dev and cam_sim_alloc_dev().

Add cam_sim_alloc_dev() as a wrapper to cam_sim_alloc() which takes
a device_t instead of the unit_number (which we can derive from the
dev again).

Add device_t sim_dev to struct cam_sim. It will be used to pass through
the bus for cases when both sides of CAM speak newbus already and we want
to link them (yet make the calls through CAM for now).

SDIO will be the first consumer of this. For that make use of
cam_sim_alloc_dev() in sdhci under MMCCAM.

This will also allow people to start iterating more on the idea
to newbus-ify CAM without changing 50+ device drivers from the start.
Also to be clear there are callers to cam_sim_alloc() which do not
have a device_t (e.g., XPT) or provide their own unit number so we cannot
simply switch the KPI entirely.

Submitted by: kibab (original idea, see https://reviews.freebsd.org/D12467)
Reviewed by: imp, chuck
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19746