aarch64: reset: initialize CNTFRQ_EL0 in the highest EL

Can only be written at the highest Exception level implemented.
For example, if EL3 is the highest implemented Exception level,
CNTFRQ_EL0 can only be written at EL3.

Also move z_arm64_el_highest_plat_init to be called when is_el_highest

Signed-off-by: Peng Fan <peng.fan@nxp.com>