dt-bindings: memory-controllers: exynos5422-dmc: Add interrupt mode

Add description for optional interrupt lines. It provides a new
operation mode, which uses internal performance counters interrupt when
overflow.  This is more reliable than using default polling mode
implemented in devfreq.

Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>