boards: nucleo_l073rz: Provide working PM configuration

This commit provides required bits for working PM configuration on this
board:
- Enable LPTIM node (status = "okay")
- Configure SYS_CLOCK_TICKS_PER_SEC value to be compatible with
LPTIM source clock (LSI: 32KHz)
- Configure an exit-latency which is compatible with execution of
tests/kernel/timer/timer_api/tickless tests
- Increase min-residency-us to be higher than exit-latency

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