powerpc: Mark powermac as orphan in MAINTAINERS

Ben no longer has time to do any maintenance of the powermac code. Mark
it as orphan.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230517074819.52546-1-mpe@ellerman.id.au

MAINTAINERS

@@ -11907,9 +11907,8 @@ F:	lib/linear_ranges.c
 F:	lib/test_linear_ranges.c
 
 LINUX FOR POWER MACINTOSH
-M:	Benjamin Herrenschmidt <benh@kernel.crashing.org>
 L:	linuxppc-dev@lists.ozlabs.org
-S:	Odd Fixes
+S:	Orphan
 F:	arch/powerpc/platforms/powermac/
 F:	drivers/macintosh/
 X:	drivers/macintosh/adb-iop.c