boards: nrf52840_pca10056: Rename board to nrf52840dk_nrf52840

The nRF52840 DK board target, so far known as nrf52840_pca10056,
is renamed to nrf52840dk_nrf52840.
Its documentation and all references to its name in the tree are
updated accordingly. Overlay and configuration files specific to
this board are also renamed, to match the new board name.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>