[modules] geo_mag cleanup and doc improvements
[stm32][adc] fix for f1

- update libopencm3 to get adc_set_injected_sequence fix for f1
- set_sample_time_on_all_channels rather than on each channel separately since we use the same sample time for all anyway
- also use some convenience functions instead of directly reading the registers

- since compatibility with old systick was removed in libopencm3 with commit bfc65271c083ad2cd79e61901a769d8c354d863b,
  also update the systick clock source in stm32 sys_time_arch and luftboot