Using SDMA1 with UART1 is causing a "Timeout waiting for CH0" error.
This patch changes to ahb clock from SDMA1_ROOT to AHB which
fixes the timeout error.

Fixes:  a05ea40e ("arm64: dts: imx: Add i.mx8mm dtsi support")
Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>