arm:exynos: add common DTS file for exynos 4

This patch adds common dtsi file and config header for all
Exynos 4 based boards.

Patch additionaly adds board specific (weak) functions for
board_early_init_f and board_power_init functions.

Signed-off-by: Piotr Wilczek <p.wilczek@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>