ARM: dts: Use labels for overriding nodes in exynos542x

Usage of labels instead of full paths reduces possible mistakes when
overriding nodes. Additionally remove duplicated serial and uart labels
for serial.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>