ARM64: dts: bcm2837-rpi-3-b: remove incorrect pwr LED

We are incorrectly defining the pwr LED, attaching it to a gpio line
that is wired to the Wi-Fi SDIO module (which fails due to this).

The actual power LED is connected to the GPIO expander, which we don't
expose currently.

Fixes: 9d56c22a ("ARM: bcm2835: Add devicetree for the Raspberry Pi 3.")
Thanks-to: Eric Anholt <eric@anholt.net> [for clarifying we can't control the LED]
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>