ARM: dts: exynos: Enable DMA support for UART modules on Exynos5 SoCs

UART modules can use DMA for offloading data transfers and reducing
interrupts, so enable this feature for Exynos5 boards. Tested on
Google ChromeBook Snow (Exynos5250), Odroid XU (Exynos5410) and Odroid
XU3 (Exynos5422) boards.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>