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.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/ep93xx_dma.c

@@ -147,6 +147,7 @@ struct ep93xx_dma_desc {
  *                is set via .device_config before slave operation is
  *                prepared
  * @runtime_ctrl: M2M runtime values for the control register.
+ * @slave_config: slave configuration
  *
  * As EP93xx DMA controller doesn't support real chained DMA descriptors we
  * will have slightly different scheme here: @active points to a head of
@@ -187,6 +188,7 @@ struct ep93xx_dma_chan {
  * @dma_dev: holds the dmaengine device
  * @m2m: is this an M2M or M2P device
  * @hw_setup: method which sets the channel up for operation
+ * @hw_synchronize: synchronizes DMA channel termination to current context
  * @hw_shutdown: shuts the channel down and flushes whatever is left
  * @hw_submit: pushes active descriptor(s) to the hardware
  * @hw_interrupt: handle the interrupt