ARM: mackerel: include the correct .dtsi file

Mackerel's .dts Device Tree description file should derive from the SoC's
.dtsi, not from skeleton.dtsi directly.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Simon Horman <horms@verge.net.au>