cros: exynos: enable cros-ec for smdk5250

This patch initiates cros-ec in board_init() to enable it for smdk5250.

This patch depends on the patch in the MMC series that brings in exynos5-dt.c.
Refer to http://patchwork.ozlabs.org/patch/240084

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Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Signed-off-by: Hung-ying Tyan <tyanh@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>