dt-bindings: fix dangling Documentation/arm64 reference

The arm64 documentation has move under Documentation/arch/ fix a reference
to match.

Cc: Krzysztof Kozlowski <[email protected]>
Cc: Conor Dooley <[email protected]>
Cc: [email protected]
Acked-by: Catalin Marinas <[email protected]>
Acked-by: Rob Herring <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>

Author: Jonathan Corbet

Documentation/devicetree/bindings/cpu/idle-states.yaml

@@ -259,7 +259,7 @@ description: |+
       http://infocenter.arm.com/help/index.jsp
 
   [5] ARM Linux Kernel documentation - Booting AArch64 Linux
-      Documentation/arm64/booting.rst
+      Documentation/arch/arm64/booting.rst
 
   [6] RISC-V Linux Kernel documentation - CPUs bindings
       Documentation/devicetree/bindings/riscv/cpus.yaml