dmaengine: ep93xx_dma: Provide some missing struct attribute documentation

dmaengine: ep93xx_dma: Provide some missing struct attribute documentation

A couple of entries were missed, causing kerneldoc to complain.

Fixes the following W=1 kernel build warning(s):

 drivers/dma/ep93xx_dma.c:183: warning: Function parameter or member 'slave_config' not described in 'ep93xx_dma_chan'
 drivers/dma/ep93xx_dma.c:215: warning: Function parameter or member 'hw_synchronize' not described in 'ep93xx_dma_engine'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Lennert Buytenhek <buytenh@wantstofly.org>
Cc: Ryan Mallon <rmallon@gmail.com>
Cc: Mika Westerberg <mika.westerberg@iki.fi>
Link: https://lore.kernel.org/r/20200714111546.1755231-4-lee.jones@linaro.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>