ARM: dts: Rename Exynos5250 ChromeOS common file to have exynos prefix

Make it easier to notice the common file for ChromeOS devices based on
the Exynos5250 by giving it the exynos5250 prefix that the boards have.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Tomasz Figa <t.figa@samsung.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>