MAINTAINERS: Update ps3vram block driver

Add myself as co-maintainer of the ps3vram block driver, and add linuxppc-dev
as a relevant mailing list.

I have been acting as maintainer of this driver for the last several years, and
if there is some inquiry regarding it I would like to be notified.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Acked-by: Jim Paris <jim@jtan.com>
Signed-off-by: Jens Axboe <axboe@fb.com>