dt-bindings: fix dangling Documentation/arm64 reference

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

Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
Cc: Conor Dooley <conor+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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