boards: nrf52840 common partition dtsi

Add common default flash partition layout for nrf52840 as many boards
have used identical flash layouts.

The default flash layout was updated to remove scrach in 2022 (9a842588)
but almost all boards were still using the previous layout, so this
updates them to the new layout with allows for larger applications.

This commit also incorporates feeedback from @nordicjm in PR #77791 to
change slot0 to 0x00077000 and slot1 to 0x00075000: "If you use swap
using move, you need a sector for the data to be moved up by, and you
need space for the swap status fields, which is about a sector, so by
making the changes here you get the full 0x65000 for an image, without
these changes you get 0x64000.

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