MAINTAINERS: Exclude m68k-only drivers from powerpc entry

The powerpc section has a "F:" entry for drivers/macintosh, matching all
files in or below drivers/macintosh. That is correct for the most part,
but there are a couple of m68k-only drivers in the directory, so exclude
those.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230531125023.1121060-1-mpe@ellerman.id.au