drivers: counter: rtc_stm32: convert to DT_INST defines

Convert driver to use DT_INST_ defines.
Removed DT_RTC_0 fixup macros but keep DT_RTC_NAME_0 as it is
still in use across RTC users.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>