of: add vendor prefix for GMT

Adding Global Mixed-mode Technology Inc. to the list
of devicetree vendor prefixes.

Signed-off-by: Wei Ni <wni@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>