dmaengine: Move kdoc description of struct dma_chan_percpu closer to it

We have split by unknown reason of kdoc and struct dma_chan_percpu definition.
Join them back. No functional change.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210518104323.37632-1-andriy.shevchenko@linux.intel.com


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