The ARM generic timer keeps ticking even if disabled or it expired.
In case of updating it with a very low value it might expire again
after writing the tval but before updating ctrl. In that case we do
lose the status bit saying that the timer expired and we will consequently
not get an interrupt for it, leaving the timer in a "dead" state.

In order to solve this increase the minimum period with what the timer
can be loaded to something higher.

Found & analysed with: gem5
Debugged with: andrew
Sponsored by: DARPA/AFRL
Reviewed by: andrew
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D6202