On i.MX8MN, usdhc's ipg clock is from IMX8MN_CLK_IPG_ROOT,
assign it explicitly instead of using IMX8MN_CLK_DUMMY.

Fixes: 6c3debcb ("arm64: dts: freescale: Add i.MX8MN dtsi support")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>