boards: arty: add support for daplink qspi mux

The ARM Cortex-M1/M3 DesignStart FPGA reference designs support booting
from an off-board, memory-mapped QSPI NOR flash device. The V2C DAPlink
shield from ARM Ltd. provides support for this feature.

If the board/shield is not configured for booting from the QSPI NOR
device, that same device is available as a regular QSPI NOR flash
device.

The presense of a shield configured for QSPI NOR flash boot is indicated
through an IRQ line used as a level-detect, non-interrupt signal.

Introduce a board specific devicetree binding for the DAPLink QSPI MUX
and provide a board specific API accessing it. Automatically set the
QSPI MUX to to provide regular QSPI NOR flash access if the board/shield
is not configured for memory-mapped QSPI NOR flash boot.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>