ARM: dts: fork out common EXYNOS5 nodes

In preparation of adding support for EXYNOS5420, which has many
peripherals similar to EXYNOS5250, a new common EXYNOS5 device tree
source file is created out of the exising EXYNOS5250 device tree
source file. Only the common nodes required for basic boot up on
EXYNOS5420 based boards are moved into this new file and the rest
of the common nodes would be moved subsequently.

EXYNOS5440 SoC is quite different from EXYNOS5250 and EXYNOS5420.
Hence it is not possible to reuse "exynos5.dtsi" for EXYNOS5440.

Signed-off-by: Chander Kashyap <chander.kashyap@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>