net/cadence_gem: Implement SAR (de)activation

net/cadence_gem: Implement SAR (de)activation

The Specific address registers can be enabled or disabled by software.
QEMU was assuming they were always enabled. Implement the
disable/enable feature. SARs are disabled by writing to the lower half
register. They are re-enabled by then writing the upper half.

Reported-by: Deepika Dhamija <deepika@xilinx.com>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 49efd1f7450af8f980b967d3054245bae137866c.1386136219.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>