dma/imx-sdma: use readl_relaxed/writel_relaxed and use writel when necessary

use readl_relaxed/writel_relaxed in most places, and use writel when
enable channel, because it needs memory barrier.

Signed-off-by: Richard Zhao <richard.zhao@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>