boards: arm: nrf52840dongle_nrf52840: Fix boot partition address

Fixes an issue when mcuboot is built for this board with the nRF5x
SDK bootloader Kconfig disabled to allow it to be placed at 0x0
instead of 0x1000 whereby it would not boot.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>