ARM: dts: exynos: Enable UART3 on Exynos5410

Just like other Exynos5 family SoCs, this one has four UARTs. Configure
clocks for UART3 and enable it.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>