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