Change i.MX6/i.MX7 SoCs usdhc node name from usdhc to mmc to be
compliant with yaml schema, it requires the nodename to be "mmc".
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>