dma: imx-dma: use devm_kzalloc and devm_request_irq

Use managed functions devm_kzalloc and devm_request_irq instead to
make the error patch simpler.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Vinod Koul <vinod.koul@linux.intel.com>