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

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

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20240123083450.20996-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>