counter: stm32-lptimer-cnt: Add const qualifier for functions_list array

counter: stm32-lptimer-cnt: Add const qualifier for functions_list array

The struct counter_count functions_list member expects a const enum
counter_count_function array. This patch adds the const qualifier to the
stm32_lptim_cnt_functions to match functions_list.

Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Link: https://lore.kernel.org/r/08830119594acdb3bc9d36ae2e65b2eb26589ade.1623201081.git.vilhelm.gray@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>