dmaengine: ste_dma40: Fetch the number of physical channels from DT

Some platforms insist on obscure physical channel availability. This
information is currently passed though platform data in internal BSP
kernels. Once those platforms land, they'll need to configure them
appropriately, so we may as well add the infrastructure.

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>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>