On i.MX8MQ, usdhc's ipg clock is from IMX8MQ_CLK_IPG_ROOT,
assign it explicitly instead of using IMX8MQ_CLK_DUMMY.

Fixes: 748f908c ("arm64: add basic DTS for i.MX8MQ")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>