brd: update maintainer to be Jens Axboe

Nick Piggin is currently listed as the maintainer of BRD in MAINTAINERS,
but the mails sent to the listed address are returned as undeliverable.
Update the maintainer for BRD to be Jens Axboe, since patches for BRD
flow up through him.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>