dt-bindings: timer: narrow regex for unit address to hex numbers

Regular expression used to match the unit address part should not allow
non-hex numbers.  Expect at least one hex digit as well.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240325104833.33372-2-krzysztof.kozlowski@linaro.orgSigned-off-by: Rob Herring <robh@kernel.org>

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

@@ -60,7 +60,7 @@ properties:
       be implemented in an always-on power domain."
 
 patternProperties:
-  '^frame@[0-9a-z]*$':
+  '^frame@[0-9a-f]+$':
     type: object
     additionalProperties: false
     description: A timer node has up to 8 frame sub-nodes, each with the following properties.