MAINTAINERS: Remove Javier Martinez Canillas as reviewer for Exynos

I left Samsung and lost access to most Exynos hardware and documentation.
Also, I likely won't be able to keep an eye on the platform anymore in the
short term so remove myself as a reviewer for Exynos.

Signed-off-by: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>