MAINTAINERS: powerpc: Add Ritesh and Shrikanth

Ritesh and Shrikanth has been helping in the powerpc mailing list
patch reviews, adding them as reviewers.

Acked-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Acked-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Acked-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260818035037.613186-1-maddy@linux.ibm.com