maintainers: Add myself as a OpenBSD maintainer

Add myself as an OpenBSD maintainer and add OpenBSD as maintained.

Signed-off-by: Brad Smith <brad@comstyle.com>
Reviewed-by: Kamil Rytarowski <n54@gmx.com>
Message-id: 20180216164620.GA53727@humpty.home.comstyle.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>