ARM: zynq: Rename bus to be align with simple-bus yaml

Rename amba to AXI. Based on Xilinx Zynq TRM (Chapter 5) chip is "AXI
point-to-point channels for communicating addresses, data, and response
transactions between master and slave clients. This ARM AMBA 3.0..."

Issues are reported as:
.. amba: $nodename:0: 'amba' does not match
'^([a-z][a-z0-9\\-]+-bus|bus|soc|axi|ahb|apb)(@[0-9a-f]+)?$'
>From schema:
../github.com/devicetree-org/dt-schema/dtschema/schemas/simple-bus.yaml

Similar change has been done for Xilinx ZynqMP SoC.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/8a4bc80debfbb79c296e76fc1e4c173e62657286.1606397101.git.michal.simek@xilinx.com

arch/arm/dts/zynq-7000.dtsi

@@ -95,7 +95,7 @@
 		};
 	};
 
-	amba: amba {
+	amba: axi {
 		u-boot,dm-pre-reloc;
 		compatible = "simple-bus";
 		#address-cells = <1>;