boards: arm: b1_l072z_lrwan1: Use LSE for RTC
boards: arm: b1_l072z_lrwan1: Use LSE for RTC

The module has a 32768Hz crystal for RTC so let's use it.

Signed-off-by: Serafin Leschke <1a89d41b3fd75f3c8f10dfba4bf3720bbb7f4e9d@bytesatwork.ch>

zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr