ARM: dts: exynos: Fix DTC unit name warnings in SMDK5420

Fix following DTC warnings in Exynos5420 SMDK5420:

Warning (unit_address_vs_reg): Node /dp-controller@145B0000/display-timings/timing@0 has a unit name, but no reg property

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>