arm64: dts: imx8mn: Change SDMA1 ahb clock for imx8mn

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: 6c3debcb ("arm64: dts: freescale: Add i.MX8MN dtsi support")

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>