dma: pl330: Implement device_slave_caps

Implement the device_slave_caps() callback for the pl330 driver. This allows
dmaengine users like the generic ALSA dmaengine PCM driver to query the
capabilities of the driver. The PL330 supports all buswidths and both
mem-to-dev as well as dev-to-mem transfers. In theory there is no limit on the
number of segments that can be transferred (in practice you'll run out of memory
eventually) and the number of bytes per segment is limited by the size of the
PL330 program buffer. Due to the nature of the PL330 the maximum number of bytes
per segment depends on the burstsize, the driver sets it to the value for a
1-byte burstsize, since it is the smallest.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>