dmtimer(4): initialize stathz, profhz as other drivers, platforms do

dmtimer(4): initialize stathz, profhz as other drivers, platforms do

Don't hardcode stathz to 100 and profhz to 1000.  Instead, set stathz
to hz and profhz to (stathz * 10).  This is what we do in all other
armv7 clock interrupt drivers and most other platforms.

Link: https://marc.info/?l=openbsd-tech&m=167479021815637&w=2
ok kettenis@