dmaengine: ste_dma40: support fixed physical channel allocation

This patch makes existing use_fixed_channel field (of stedma40_chan_cfg
structure) applicable to physical channels.

Signed-off-by: Gerald Baeza <gerald.baeza@stericsson.com>
Tested-by: Yannick Fertre <yannick.fertre@stericsson.com>
Reviewed-by: Per Forlin <per.forlin@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>