dmaengine: stm32-dmamux: add suspend/resume power management support

Add suspend/resume power management relying on PM Runtime engine.

Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/20200128094158.20361-2-amelie.delaunay@st.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>