ARM: dts: exynos: Add exynos5420-fimd compatible

This patch changes the compatible of Exynos5420 fimd
to "exynos5420-fimd". To support MIC bypass from display
path, the new compatible is introduced for Exynos5420.

Cc: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>