dmaengine: ste_dma40: indicate directions on channels

Since the introduction of the .directions flags, ste_dma40 was
never patched to indicate which transfer directions it can manage.
This causes a problem when trying to use the dmaengine for generic
ALSA SoC DMA:

ux500-msp-i2s.1: Failed to get DMA channel capabilities, falling
   back to period counting: -6

This patch fixes this issue by indicating the supported transfer
directions for slave and memcpy channels.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>