MAINTAINERS: update amdkfd maintainer (v3)

I'm leaving the role of amdkfd maintainer. Therefore, update the relevant
entry in the MAINTAINERS file with the name of the new maintainer.

Good Luck!

v3: update mailing list, file list (Alex)

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com> (v2)
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>