MAINTAINERS: add Johan to Bluetooth maintainers

I've been coordinating Bluetooth patches in my tree for some time and
it's possible I'll do it in the future too, so add myself to the
Bluetooth sections as well as mention my tree there.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: "Gustavo F. Padovan" <padovan@profusion.mobi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>