Fix a typo in the TX DMA interrupt name for AUART4.
This patch makes AUART4 operational again.

Signed-off-by: Marek Vasut <marex@denx.de>
Fixes: f30fb03d ("ARM: dts: add generic DMA device tree binding for mxs-dma")
Cc: stable@vger.kernel.org
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>