mpr: Add workaround for too few slots being automatically scanned

This patch adds a /boot/loader.conf setting that makes it possibly to
override the detected number of slots in storage enclosures. Some (yes
I'm looking at you HPE D6020!) reports less available slots that there
actually are (the D6020 seems to report 18 but actually has 35 per
drawer). This causes the mpr driver to have problems detecting/managing
all drives in a multienclosure setting. For the D6020 this occurs when
connecting two or more fully equipped (140 drives) enclosures to one
controller...

This problem can be "fixed" by adding the following to /boot/loader.conf
and rebooting:

hw.mpr.encl_min_slots="35"

Note: I (Warner) don't have this hardware to see if there's some way to
fix the detection, so I'm committing this as a stop-gap. It's a no-op if
no tunable is set.

PR: 271238
Reivewed by: imp