MAINTAINERS: Add myself as reviewer for Samsung Exynos support

I've been helping reviewing and testing Exynos SoC support patches
for the last couple of years. But it would be easier for me if I'm
cc'ed for patches, so I'm adding myself as reviewer for this entry.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Kukjin Kim <kgene@kernel.org>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>