Update maintainers for powerpc

Update maintainers for powerpc

This updates the MAINTAINERS entries for powerpc.  It adds Ben H to
the overall Linux for PowerPC entry and makes it clear this covers
both 32-bit and 64-bit machines.  It removes the separate entry we had
for Linux on 64-bit PowerPC where Anton and I were listed as
maintainers - Anton hasn't been involved in the day-to-day maintenance
of the code for several years.  Finally, it removes the entry for the
Linux for PowerPC boot code where Tom Rini was listed as the
maintainer.  That code got completely rewritten when we merged
32-bit and 64-bit, and I and the various platform maintainers have
been maintaining that code since.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Acked-by: Tom Rini <trini@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>