MAINTAINERS: add docs entry to AMDGPU

To make sure maintainers of amdgpu drivers are aware of any changes
 in their documentation, add its entry to MAINTAINERS.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tales Lelo da Aparecida <tales.aparecida@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>