dmaengine/dw_dmac: Add 64 bit access width support for slave xfers on mem side

64 bit transfers are possible on both sides in slave transfers (memory as well
as peripherals). This patch adds support for it memory side 64 bit transfers.

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