dt-bindings: Rename Exynos to lowercase

Fix up inconsistent usage of upper and lowercase letters in "Exynos"
name.

"EXYNOS" is not an abbreviation but a regular trademarked name.
Therefore it should be written with lowercase letters starting with
capital letter.

The lowercase "Exynos" name is promoted by its manufacturer Samsung
Electronics Co., Ltd., in advertisement materials and on website.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>