ARM: dts: at91: nattis: drop three indentation levels

Make use of the recently added &pinctrl and &watchdog labels. This
makes the whole file consistent and knowledge of the ahb/apb structure
is hidden.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>