dmaengine: Add flow controller information to dma_slave_config

Flow controller is programmable for few controllers and there are few
intelligent peripherals like, Synopsys JPEG controller, that needs to be a flow
controller of DMA transfers on dest side.

For this, currently two drivers, pl08x and dw_dmac, support flow controller to
be passed from platform to these drivers.

Perhaps, this should be a part of struct dma_slave_config. This patch adds
another field device_fc to this structure. User drivers must pass this as true
if they want to be flow controller of certain transfers.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>