dmaengine: dw_dma: add Device Tree probing capability

SPEAr platforms now support DT and so must convert all drivers to support DT.
This patch adds DT probing support for Synopsys DMA controller and updates its
documentation too.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>