ARM: dts: imx7: add USDHC NAND and IPG clock to SDHC instances

The USDHC instances need the USDHC NAND and IPG clock in order to
operate. Reference them properly by replacing the dummy clocks with
the actual clocks.

Note that both clocks are currently implicitly enabled since they
are part of the i.MX 7 clock drivers init_on list. This might
change in the future.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>