MAINTAINERS: Add Sasha as a stable branch maintainer

Sasha has somehow been convinced into helping me with the stable kernel
maintenance.  Codify this slip in good judgement before he realizes what
he really signed up for :)

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>