dmaengine: ste_dma40: physical channels number correction

DMAC_ICFG[0:2]=SCHNB only allows to count 'multiple of 4' physical
channels so it was ok with platforms having 8 channels but cannot be
used for next versions (with 10 or 14 channels).  This patch allows to
provide the number of physical channels for a DMA device via
platform_data, or still rely on SCHNB if platform_data announces 0
channel.

Signed-off-by: Gerald Baeza <gerald.baeza@stericsson.com>
Reviewed-by: Per Forlin <per.forlin@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>