MAINTAINERS: Add btrfs mailing list and myself as reviewer

Since the current code base is mostly from btrfs-progs, anyone
contributing to U-Boot btrfs code could also help us to improve
btrfs-progs and btrfs kernel module.

Also add myself as designated reviewer.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>