MAINTAINERS: powerpc: Remove myself as a reviewer

I haven't been able to participate and help with this as much as I had
hoped, and it doesn't look like I will be able to spend time on powerpc
going forward.

Signed-off-by: Naveen N Rao (AMD) <naveen@kernel.org>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250420163248.1746379-1-naveen@kernel.org