F09x and L4 share the same cselr register, as well as some L0s, factorize definitions in a new shared header and add helpers. fyi, that register allows to redefine dma channel peripheral mapping - see device datasheet for mapping tables.
Far from complete support for the channel selection systems on f0/l4, but at least brings in the defines needed for doing this yourself. Fixes https://github.com/libopencm3/libopencm3/issues/1001
Same as L1 according to L1-L4 migration guide, untested