drivers: dma: sdma: Make access to DMA channel stats atomic

DMA channel stats like pending_length or free is not protected
and can be modified in parallel by a consumer and a producer.

This can result in non-atomic updates which in turn will result
in using stale data.

Fix this by making regions of code accessing dma stats atomic.

Fixes: e94c86f3 ("drivers: dma: Add initial support for NXP SDMA")
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>