counter: cmsdk_apb_timer: Use clock freq from DT clocks

Previously, the CMSDK APB 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 and issues a build-time
warning.

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