MAINTAINERS: Include Samsung SoC serial driver in Samsung SoC entry

Samsung SoC (S3C, S5P and Exynos) serial driver does not have dedicated
reviewing person so some patches might be missed be Samsung-related
folks (e.g. not even reaching Samsung SoC mailing list).  Include them
in generic Samsung SoC maintainer entry to provide some level of
reviewing and care.  This will not change handling of patches (via
serial tree).

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>