Merge from stable/12 r352735 and r352741

Merge all the stability fixes for the mpr and mps drivers. This fixes a
number of different panics. Unfortunately, mps now requires atomic_swap_64
to work properly, so it has been disabled on 32-bit powerpc and mips. The
impact should be negligible, however, since this device is difficult to
attach to those platforms.

Approved by: re@ (glen)
Relnotes: YES (for ppc32 removal)