ARM: ux500: Replace ST-E's home-brew DMA direction definition with the generic one

STEDMA40_*_TO_* direction definitions are identical in all but name to
the pre-defined generic DMA_*_TO_* ones. Let's make things easy by not
duplicating such things.

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