armv8/fsl-layerscape: fdt: avoid incorrect fixing with CONFIG_SYS_CLK_FREQ

Current sysclk fixing would fix all clocks with 'fixed-clock' compatible.
This patch is to fix sysclk by path to avoid any incorrect fixing.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>