ARM: dts: Extend exynos5420-pinctrl nodes using labels instead of paths

A previously defined Device Tree node, can be extended either by defining
a node using the same full path or by creating a label for the node and
referencing to it.

Using full paths is more error prone since if there was a typo error, a
new node will be created instead of extending the node as it was desired.
This will lead to run-time errors that could be hard to detect.

A mistyped label on the other hand, will cause a dtc compile error which
makes it easier to detect the mistake since happens at build-time instead.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>