ARM: dts: bcm2837-rpi-3-b-plus: Fix GPIO line name of power LED

The red LED on the Raspberry Pi 3 B Plus is the power LED.
So fix the GPIO line name accordingly.

Fixes: 71c0cd22 ("ARM: dts: bcm2837: Add Raspberry Pi 3 B+")
Signed-off-by: Phil Elwell <phil@raspberrypi.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>