boards: nrf52840dk_nrf52840: document suppported QSPI opcodes

The SPI NOR device on this board supports fastread, 2READ, DREAD,
4READ, and QREAD for read opcodes, but only PP and QPP for write
opcodes.  Provide a clue to people trying to use 2READ with 2PP: it
won't work.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>