ARM: dts: exynos: Remove DMA controller bus node name to fix dtschema warnings

There is no need to keep DMA controller nodes under AMBA bus node.
Remove the "amba" node to fix dtschema warnings like:

    amba: $nodename:0: 'amba' does not match '^(bus|soc|axi|ahb|apb)(@[0-9a-f]+)?$'

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