boards: nrf52840 common uf2 partition

Common uf2 partition configurations to avoid duplication in boards.

There appears to be a bit of confusion about exactly what a proper
UF2 partition map looks like for the nrf52840, so common dts
configurations should help to avoid confusion.

Configuration for SoftDevice v6 and v7 provided as thats what was
fouond in use in tree already.

Signed-off-by: Jacob Winther <jacob@9.nz>