target/arm: Add a timer to predict PMU counter overflow

Make PMU overflow interrupts more accurate by using a timer to predict
when they will overflow rather than waiting for an event to occur which
allows us to otherwise check them.

Signed-off-by: Aaron Lindsay <aaron@os.amperecomputing.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190124162401.5111-3-aaron@os.amperecomputing.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>