dmaengine: ste_dma40: Amalgamate DMA source and destination channel numbers

Devices which utilise DMA use the same device numbers for transmitting
and receiving. In this patch we encode the source and destination
information into one single attribute. We can subsequently exploit the
direction attribute to see which of the transfer directions are being
described. This also lessens the burden on platform data.

Cc: Dan Williams <djbw@fb.com>
Cc: Per Forlin <per.forlin@stericsson.com>
Cc: Rabin Vincent <rabin@rab.in>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>