efifb: support AMD Radeon HD 6490

This patch enables the framebuffer for the AMD Radeon 6490 found in the new MacBook Pro 8,2 generation.
The framebuffer's base is located at 0x90010000, the method for obtaining it was found in the same way mentioned in https://patchwork.kernel.org/patch/91704/

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Signed-off-by: Jonathan Gonzalez <zeus@gnu.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>