dmaengine: ste_dma40: Allow memcpy channels to be configured from DT

At this moment in time the memcpy channels which can be used by the D40
are fixed, as each supported platform in Mainline uses the same ones.
However, platforms do exist which don't follow this convention, so
these will need to be tailored. Fortunately, these platforms will be DT
only, so this change has very little impact 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>