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

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

Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Link: https://lore.kernel.org/r/20240123083517.21091-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>