exynos: Add common board code for exynos5 boards that use device tree

exynos: Add common board code for exynos5 boards that use device tree

Some boards use device tree for almost all board-specific configuration.
They therefore do not need their own separate board code, but can all use
the same version. Add a common version of the board code. It uses the
PMIC, regulator and video bridge uclasses. This will support smdk5250,
smdk5420, snow, spring, pit and pi.

Signed-off-by: Simon Glass <sjg@chromium.org>