clk: imx8mn: Add M7 core clock

Add missing M7 core clock entry to the iMX8MN clock driver.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Cc: Abel Vesa <abel.vesa@nxp.com>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Link: https://lore.kernel.org/r/20210819202036.2084782-1-marex@denx.de
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>