boards: arm: b_l072z_lrwan1: Fix flashing of big firmware

OpenOCD currently uses a single-bank STM32 configuration for the
B_L072Z_LRWAN1 board. This causes flashing to fail when the firmware
image is larger than the first bank. Switch to the dual bank
configuration to make this work.

Signed-off-by: Andreas Sandberg <andreas@sandberg.pp.se>