memory: samsung: exynos5422-dmc: Add module param to control IRQ mode

The driver can operate in two modes relaying on devfreq monitoring
mechanism which periodically checks the device status or it can use
interrupts when they are provided by loaded Device Tree. The newly
introduced module parameter can be used to choose between devfreq
monitoring and internal interrupts without modifying the Device Tree.
It also sets devfreq monitoring as default when the parameter is not set
(also the case for default when the driver is not built as a module).

Reported-by: Willy Wolff <willy.mh.wolff.ml@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>