arm64: dts: allwinner: Pine64-LTS: Add status LED

The Pine64-LTS board features a blue status LED on pin PL7.

Describe it in the DT.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20210113152630.28810-4-andre.przywara@arm.com