ARM: dts: at91: nattis: set the PRLUD and HIPOW signals low

AT91_PINCTRL_OUTPUT_VAL(0) without AT91_PINCTRL_OUTPUT is a no-op, so
make sure the pins really output a zero.

Fixes: 0e432389 ("ARM: dts: at91: add devicetree for the Axentia Nattis with Natte power")
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>