boards: nrf52840dk_nrf52840: Fix reserved GPIO lines

This is a follow-up to commit 7a83724e.

There is no reason to mark that many GPIO lines as reserved on this
board. And doing so causes several existing tests to fail as they
are configured to use some of those now unavailable GPIO lines.

Limit reservation to the lines that actually cannot be used as
GPIOs without changes in the default configuration of the board
or its physical modification (via solder bridges), i.e.:
- XL1 and XL2 (connections for the 32.768 kHz crystal)
- NFC1 and NFC2 (NFC antenna connections)
- RESET
- TXD and RXD (lines used by the console UART)
- QSPI lines: CS, CLK, and DIO0-3

Provide names for all the GPIO lines that are described on the board.
Even for the reserved ones, so that it is clear why they are reserved.

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