boards: nrf52811_pca10056: Rename to nrf52840dk_nrf52811

The board target for emulation of nRF52811 on nRF52840 DK, so far
known as nrf52811_pca10056, is renamed to nrf52840dk_nrf52811.
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>