boards: swan_r5: fix discharge gpio configuration

The 3V3 bus seems to have a resistor connected to an MCU pin to
discharge the bus by pulling the discharge line low while in open-drain
mode.

The board code was using non-existing GPIO flags (see previous commit
for more details). This patch makes it clear that this GPIO is for
discharging and uses GPIO attributes appropriately. Since the regulator
is enabled by default, discharging is disabled.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>