exynos5: fix build break by adding CONFIG_POWER

Move the configs listed below from exynos5-dt-common.h to exynos5-common.h:
- CONFIG_POWER
- CONFIG_POWER_I2C
fixes build break for Arndale and Smdk5250 boards.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>