ARM: dts: bcm283x: drop alt3 from &gpio

As the alt3 group has no pins left drop it from &gpio.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>