dmaengine: ste_dma40: Assign memcpy channels in the driver

The channels reserved for memcpy are the same for all currently
supported platforms. With this in mind, we can ease the platform
data passing requirement by moving these assignments out from
platform code and place them directly into the driver.

Acked-by: Vinod Koul <vnod.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>