Update locm3 to get usb control callback fix for luftboot

Reason is just to get the fix for usb control callbacks so luftboot works again:
https://github.com/libopencm3/libopencm3/commit/922d8f7079efd232c736e84df49214cec13fb9ea

Since rcc_ppre1_frequency and rcc_ppre2_frequency were also renamed in the meantime, make appropriate changes.