ARM: dts: bcm2837-rpi-3-b: Use consistent label for HDMI hotplug

This make the GPIO label for HDMI hotplug more consistent to the other
boards.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>