dmaengine: imx-sdma: Declare slave capabilities for the generic code

Since ecc19d17

 ("dmaengine: Add a warning for drivers not using the
generic slave caps retrieval") the following warning is observed:

[    0.224981] ------------[ cut here ]------------
[    0.225013] WARNING: CPU: 0 PID: 1 at drivers/dma/dmaengine.c:830 dma_async_device_register+0x2a0/0x4c8()
[    0.225023] this driver doesn't support generic slave capabilities reporting

Declare the slave capabilities to avoid such warning.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>