ARM: dts: use exynos5420-dw-mshc compatible for exynos3250

There are some differences of mobile storage host between exynos3250
and exnos5250. For example, exynos3250 supports the HS400 mode, but
exynos5250 doesn't support it.
Since exynos3250 can perform the similar function with exynos5420
compatible, this patch changes the compatible from exynos5250 to
exynos5420 for mshc.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>