ARM: dts: sun7i: bananapi-m1-plus: red LED is power LED

The red led on the Bananapi M1+ is wired to light up as soon as the
board is powered, and continues to be lit while the GPIO is floating.

Rename the LED to "pwr", remove the default trigger, and mark it as
default on.

Also fix the pinmux node that was specifying a different pin for this
LED.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>