imx-base: Add virtual/libg2d to i.MX6's MACHINE_SOCARCH_FILTER

The virtual/libg2d package is i.MX6 dependant and should put every
package depending on it into the i.MX6's sub-architecture to avoid it
being included in other architectures due package feed.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>