[media] exynos-gsc: change driver compatible string

[media] exynos-gsc: change driver compatible string

As G-Scaler is going to stay unchanged across all exynos5 series
SoCs, changing the driver compatible string name to
"samsung,exynos5-gsc" from "samsung,exynos5250-gsc".
This change is as per the discussion in the devicetree forum.
http://www.mail-archive.com/devicetree-discuss@lists.ozlabs.org/msg16448.html

Signed-off-by: Shaik Ameer Basha <shaik.ameer@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>