powerpc: Remove PA SEMI MAINTAINERS entries

The PA SEMI entries have been orphaned for 3 ½ years, so fold them
into the main POWERPC entry. The result of get_maintainer.pl is more
or less unchanged.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Olof Johansson <olof@lixom.net>
Link: https://lore.kernel.org/r/20200224233146.23734-3-mpe@ellerman.id.au