imx8mm/mn: Add missing root clock entry for ARM core clock

The current implementation in arch/arm/mach-imx/cpu.c uses non-DM
code to retrieve the core clock frequency. As the root clock is not
listed we currently get:

CPU:   Freescale i.MX8MMQ rev1.0 at 0 MHz

Fix this by adding the missing entry, which results in:

CPU:   Freescale i.MX8MMQ rev1.0 at 1200 MHz

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>