boards: arm: arty: reduce number of IRQs from 8 to 7

The ARM Cortex-M1/M3 DesignStart FPGA reference designs both use IRQ 7
(the last IRQ line) as a level-detect non-interrupt signal to determine
whether the V2C-DAPLINK shield is installed. Thus IRQ 7 cannot be used
as a regular IRQ line.

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