counter: ti-ecap-capture: Utilize COUNTER_COMP_FREQUENCY macro

Reduce boilerplate by leveraging the COUNTER_COMP_FREQUENCY() macro to
define the "frequency" extension.

Link: https://lore.kernel.org/r/ZfxhEKdSi1amfcJC@ishi


Signed-off-by: William Breathitt Gray <wbg@kernel.org>