[ext][stm32] update libopencm3

to get updates from libopencm3 and remove our own workarounds for:
- STM32F4 25Mhz clock: https://github.com/libopencm3/libopencm3/pull/311
- STM32 defines for TIM9-TIM17: https://github.com/libopencm3/libopencm3/pull/350
- STM32 set I2C address: https://github.com/libopencm3/libopencm3/pull/344

additionally get fixes:
- STM32 safe gpio toggle: https://github.com/libopencm3/libopencm3/pull/337
Merge pull request #932 from paparazzi/update_locm3

[ext][stm32] update libopencm3