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

Author: rnav

MAINTAINERS

@@ -13626,7 +13626,6 @@ M:	Madhavan Srinivasan <maddy@linux.ibm.com>
 M:	Michael Ellerman <mpe@ellerman.id.au>
 R:	Nicholas Piggin <npiggin@gmail.com>
 R:	Christophe Leroy <christophe.leroy@csgroup.eu>
-R:	Naveen N Rao <naveen@kernel.org>
 L:	linuxppc-dev@lists.ozlabs.org
 S:	Supported
 W:	https://github.com/linuxppc/wiki/wiki