media: dt-bindings: s5p-mfc: Add new compatible string for Exynos3250 SoC

Since MFC v7 support was present for both Exynos5420 and Exynos3250
SoC with same compatible string "samsung,mfc-v7". As both SoCs
having different hardware properties and having same compatible
string for both SoCs doesn't seems to be correct.
Add new compatible for Exynos3250 SoC followed by mfc-v7 fallback
which will differentiate the node properties for both SoCs which
support MFC v7.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Tommaso Merciai <tommaso.merciai@amarulasolutions.com>
Suggested-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Aakarsh Jain <aakarsh.jain@samsung.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>