dt-bindings: i2c: realtek,rtl9301: Fix missing 'reg' constraint

Lists should have fixed amount if items, so add missing constraint to
the 'reg' property (only one address space entry).

Fixes: c5eda033 ("dt-bindings: i2c: Add Realtek RTL I2C Controller")
Cc: <stable@vger.kernel.org> # v6.13+
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/20250702061530.6940-2-krzysztof.kozlowski@linaro.org