Add some useful trace events for the ARM generic timers (notably
the various register writes and the resulting IRQ line state).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1476294876-12340-3-git-send-email-peter.maydell@linaro.org