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

Regular expression used to match the unit address part should not allow
non-hex numbers.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240325104833.33372-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Rob Herring <robh@kernel.org>