MAINTAINERS: add entry for AMD PP code

Add separate entry for the power managent code on AMD GPUs.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Rex Zhu <rezhu@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>