dmaengine: ste_dma40: Allow driver to be probe()able when DT is enabled

When booting using Device Tree, devices aren't registered in the normal
way. Instead, they need to be provided with a compatible string which is
held in an OF Match Table for comparison during start-up. Here we provide
the compatible string and prepare the driver to not receive a platform
data pointer.

Acked-by: Vinod Koul <vnod.koul@intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
[Fixed up whitespace error]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>