dt-bindings: timer: arm,arch_timer: Allow dual compatible string

Since the Armv7 and Armv8 architected timers are compatible, it is valid
to expose a devicetree node with compatible string "arm,armv8-timer"
followed by "arm,armv7-timer". For example a 32-bit guest running on a
64-bit machine may look for the v7 string even though the hardware is v8.
VMMs such as QEMU and kvmtool have been using this compatible string for
some time. Clean up the compatible list a little and add the dual
option.

Acked-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Jean-Philippe Brucker <[e-mail]>
Acked-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/timer/arm,arch_timer.yaml

···
   compatible:
     oneOf:
       - items:
-          - enum:
-              - arm,cortex-a15-timer
-          - enum:
-              - arm,armv7-timer
+          - const: arm,cortex-a15-timer
+          - const: arm,armv7-timer
       - items:
           - enum:
               - arm,armv7-timer
-      - items:
-          - enum:
               - arm,armv8-timer
+      - items:
+          - const: arm,armv8-timer
+          - const: arm,armv7-timer
 
   interrupts:
     minItems: 1