counter: cmsdk_apb_dualtimer: Use clock freq from DT clocks

Previously, the CMSDK APB dual timer driver hardcoded the counter
clock frequency to 24 MHz, which limits reuse across SoCs and
boards with different timer clock sources.

This patch replaces the hardcoded frequency with a value derived
from the device tree's `clocks` phandle, using the
`clock-frequency` property of the referenced clock controller node.

If the property is missing, it falls back to a default 24 MHz.

Signed-off-by: Lidor T <lidor@exibit-iot.com>