clk: imx7d: add IMX7D_MU_ROOT_CLK

This clock is needed for iMX mailbox driver

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>