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

  amba: $nodename:0: 'amba' does not match '^([a-z][a-z0-9\\-]+-bus|bus|soc|axi|ahb|apb)(@[0-9a-f]+)?$'
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Jonathan Bakker <xc-racer2@live.ca>
Link: https://lore.kernel.org/r/20200907161141.31034-6-krzk@kernel.org