MAINTAINERS: Add designated reviewers for IIO subsystem

Add those persons who generally tend to review new IIO patches to the list of
designated reviewers to make sure that they are Cc'ed on new patches.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Peter Meerwald <pmeerw@pmeerw.net>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>