arcv2_timer: fix cycle count race

It's possible the timer interrupt could occur when performing the
computation, resulting in incorrect values returned.

It's still possible for bad values to be returned if the function is
called with interrupts locked, but that is only fixable with a second
timer source.

Issue: ZEP-1546
Change-Id: I16d5b04c3e32377f7249eb4fb1bf2f7c22bd0836
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>