package/mesa3d: Enable support for Radeon r600 gallium driver

--
v2: - rebased patch from micro1183: http://patchwork.ozlabs.org/patch/398858/


    - removed support for r300 & radeonsi because they need llvm support
      which is not provided by buildroot

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>