dmaengine: mv_xor_v2: Supply some missing 'struct mv_xor_v2_device' attribute docs

dmaengine: mv_xor_v2: Supply some missing 'struct mv_xor_v2_device' attribute docs

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

 drivers/dma/mv_xor_v2.c:168: warning: Function parameter or member 'clk' not described in 'mv_xor_v2_device'
 drivers/dma/mv_xor_v2.c:168: warning: Function parameter or member 'reg_clk' not described in 'mv_xor_v2_device'
 drivers/dma/mv_xor_v2.c:168: warning: Function parameter or member 'irq_tasklet' not described in 'mv_xor_v2_device'
 drivers/dma/mv_xor_v2.c:168: warning: Function parameter or member 'hw_queue_idx' not described in 'mv_xor_v2_device'
 drivers/dma/mv_xor_v2.c:168: warning: Function parameter or member 'msi_desc' not described in 'mv_xor_v2_device'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200714111546.1755231-16-lee.jones@linaro.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>