[FIX] Add select strobe for write decoding to openMAC timer

This fix solves unwanted writes to the timer instance occurred on Xilinx
systems.

Change-Id: I6d8eb34bc300843beeda8ab9a7b053ccde7d04b4
Reviewed-on: http://brateggevoat1/gerrit/1632
Reviewed-by: Thomas Mair <thomas.mair@br-automation.com>
Tested-by: Jenkins <jenkins@brateggevoat1.br-automation.co.at>
Reviewed-by: Joerg Zelenka <joerg.zelenka@br-automation.com>