clk: imx: imx8mm: Fix the first root clock in imx8mm_ahb_sels[]

The 24MHz oscillator clock is referenced by "clock-osc-24m" and not
"osc_24m".

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>