sata highbank: add bit-banged SGPIO driver support

Highbank supports SGPIO by bit-banging out the SGPIO signals over
three GPIO pins defined in the DTB. Add support for this SGPIO
functionality.

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