board: samsung: add initial Espresso7420 board support

Espresso7420 is a development/evaluation board for Exynos7420 SoC. It
includes multiple onboard compoments (EMMC/Codec) and various
interconnects (USB/HDMI).

Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>
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