hw/intc/bcm2836_control: Implement local timer

hw/intc/bcm2836_control: Implement local timer

The BCM2836 control logic module includes a simple
"local timer" which is a programmable down-counter that
can generates an interrupt. Implement this functionality.

Signed-off-by: Zoltán Baldaszti <bztemail@gmail.com>
[PMM: wrote commit message; wrapped long line; tweaked
 some comments to match the final version of the code]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>