counter_basic_api test fails to build on STM32 platforms

[lpawelcz]

tests/drivers/counter/counter_basic_api fails to build in CI on platforms:

• stm32f103_mini
• nucleo_l4a6zg
• nucleo_f103rb

This happens on current main branch (27ec69e) and in CIs (see: https://github.com/zephyrproject-rtos/zephyr/pull/50352/checks?check_run_id=9478691624). The issue is caused by warnings about deprecated Kconfigs

To Reproduce
Steps to reproduce the behavior:

1. checkout on current main branch (tested on 27ec69e)
2. west build -p always -b nucleo_l4a6zg tests/drivers/counter/counter_basic_api
3. see preprocessor warning: #warning Kconfig COUNTER_RTC_STM32_CLOCK_LS* are deprecated. Please define clock source in dtsi file

Expected behavior
Build process for tests/drivers/counter/counter_basic_api on platforms stm32f103_mini, nucleo_l4a6zg, nucleo_f103rb should not throw any warnings.

Impact
CI failures in PRs, e.g. in #50352, CI run:https://github.com/zephyrproject-rtos/zephyr/pull/50352/checks?check_run_id=9478691624