On i.MX8MM, usdhc's ipg clock is from IMX8MM_CLK_IPG_ROOT,
assign it explicitly instead of using IMX8MM_CLK_DUMMY.

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