clk: imx7d: fix USDHC NAND clock

The USDHC NAND root clock is not gated by any CCM clock gate. Remove
the bogus gate definition.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>