mvebu: macchiatobin-singleshot: enable heartbeat LED

mvebu: macchiatobin-singleshot: enable 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 touches only Single Shot variant, because all
have revision 1.3. Some boards could be placed in an enclosure, therefore
the LED18 is enabled by default, since that'll be the only visible
indicator that the board is operating.

Ref: http://wiki.macchiatobin.net/tiki-index.php?page=Schematics#Revision1.3_Electrical_Design_Changes
Ref: http://macchiatobin.net/wp-content/uploads/2017/11/MACCHIATOBin-rev-1_3.pdf

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