arm,dev: remove PMU assertion hit on reset

Remve the assertion that we always need to add a delta larger than
zero as that does not seem to be true when we hit it in the
'PMU reset cycle counter to zero' case.

Committed by Jason Lowe-Power <power.jg@gmail.com>

src/arch/arm/pmu.cc

@@ -574,8 +574,6 @@ PMU::CounterState::add(uint64_t delta)
     const uint64_t msb(1ULL << (overflow64 ? 63 : 31));
     const uint64_t old_value(value);
 
-    assert(delta > 0);
-
     value += delta;
 
     // Overflow if the msb goes from 1 to 0