dmaengine: sprd: Use devm_ioremap_resource() to map memory

Instead of checking the return value of platform_get_resource(), we can
use devm_ioremap_resource() which has the NULL pointer check and the
memory region requesting.

Suggested-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>