devicetree: create a separate binding description for sata_highbank

The Calxeda sata_highbank driver has been adding its descriptions to the
ahci driver. Separate them properly. This patch only affects
documentation and has no functional component.

Signed-off-by: Mark Langsdorf <mark.langsdorf@calxeda.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Tejun Heo <tj@kernel.org>