dt-bindings: Add vendor prefix for SG Micro Corp

"SG Micro Corp (SGMICRO) specializes in high performance, high quality
analog IC design, marketing and sales." (http://www.sg-micro.com/

)

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Rob Herring <robh@kernel.org>