memory: samsung: exynos5422-dmc: Document mutex scope

Document scope of the mutex used by driver.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://lore.kernel.org/r/20200724180857.22119-1-krzk@kernel.org

drivers/memory/samsung/exynos5422-dmc.c

@@ -119,6 +119,7 @@ struct exynos5_dmc {
 	void __iomem *base_drexi0;
 	void __iomem *base_drexi1;
 	struct regmap *clk_regmap;
+	/* Protects curr_rate and frequency/voltage setting section */
 	struct mutex lock;
 	unsigned long curr_rate;
 	unsigned long curr_volt;