MAINTAINERS: Add documentation and dt-bindings for exynos stuff

Extend the Samsung Exynos maintainer entry to match SoC documentation
and SoC dt-bindings directories. Without that some files, like
bindings/arm/samsung/pmu.txt, are not matched by existing patterns.

This also may serve as a hint where new documentation and bindings
(not matching specific subsystem) should be put.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>