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 <[email protected]>
Signed-off-by: Peter Crosthwaite <[email protected]>
Message-id: 49efd1f7450af8f980b967d3054245bae137866c.1386136219[email protected]
Signed-off-by: Peter Maydell <[email protected]>