drivers : rtc: clean up usage of "select USE_STM32_LL"

clean up usage of select USE_STM32_LL_RCC and
select USE_STM32_LL_PWR from Kconfig.stm32
this modifications due to the include unconditionally of
stm32xxxx_ll_rcc.h and stm32xxxx_ll_pwr.h in HAL.

Signed-off-by: Khaoula Bidani <khaoula.bidani-ext@st.com>