drivers: clock_control: gd32: Refer to the CPU Frequency from the DTS

CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC is not always the same as CPU frequency.
Referring to the `/cpus/cpu@0,clock-frequency` in DTS to determine
the CPU frequency.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>