dmaengine/ste_dma40: allow fixed physical channel

Allow logical channels to specify the physical channel they want to use.
This is needed to avoid two peripherals operating on the same physical
channel during some special use-cases. (like mmc and usb during a
usb mass storage case).

Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Narayanan G <narayanan.gopalakrishnan@stericsson.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>