dt-bindings: clock: adi,axi-clkgen: add clock-output-names property

Add an optional `clock-output-names` property to the ADI AXI Clock
Generator binding. This is already being used in the Linux driver and
real-world dtbs, so we should document it to allow for correct binding
validation.

Signed-off-by: David Lechner <[e-mail]>
Link: https://lore.kernel.org/r/20250811-dt-bindings-[e-mail]
Reviewed-by: Nuno Sá <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Signed-off-by: Stephen Boyd <[e-mail]>

Documentation/devicetree/bindings/clock/adi,axi-clkgen.yaml

···
           - const: clkin2
           - const: s_axi_aclk
 
+  clock-output-names:
+    maxItems: 1
+
   '#clock-cells':
     const: 0
 
···
       reg = <0xff000000 0x1000>;
       clocks = <&osc 1>, <&clkc 15>;
       clock-names = "clkin1", "s_axi_aclk";
+      clock-output-names = "spi_sclk";
     };