exynos: Add support for spring

Spring is the first ARM-based HP Chromebook 11. It is similar to snow
and it uses the same Samsung Exynos5250 chip. But has some unusual
features. Mainline support for it has lagged snow (both in kernel and
U-Boot). Now that the exynos5 code is common we can support spring just
by adding a device tree and a few lines of configuration.

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