arm64: dts: mcbin-singleshot: add heartbeat LED

With board revision 1.3, SolidRun moved the power LED to the middle of
the board. In old place of power LED a GPIO controllable heartbeat LED
was added. This commit only touches Single Shot variant, since only this
variant is all revision 1.3.

Reported-by: Alexandra Alth <alexandra@alth.de>
Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>