add self to MAINTAINERS for the PMBus subsystem and related sensors,
and set PMBus as maintained.

Signed-off-by: Titus Rwantare <titusr@google.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Corey Minyard <cminyard@mvista.com>
Message-Id: <20220307200605.4001451-6-titusr@google.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>