ata: ahci-platform: Remove support for Exynos5440

The Exynos5440 is not actively developed, there are no development
boards available and probably there are no real products with it.
Remove wide-tree support for Exynos5440.

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