tests: drivers: counter: counter_basic_api: src: Fix STM32 RTC device

Commit fd658004 introduced a regression for the counter_basic_api test for
STM32 by using a Silabs Gecko Kconfig to enable the STM32 RTC.
This commit uses the proper STM32 Kconfig for the RTC instead.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>