ARM: dts: socfpga: Do not include skeleton.dtsi

The skeleton.dtsi file is now deprecated as noted in commit 9c0da3cc
("ARM: dts: explicitly mark skeleton.dtsi as deprecated"). The SoCFPGA
device trees already contain the nodes that are defined in skeleton.dtsi
(#address-cells, #size-cells, chosen, aliases, memory).

Including skeleton.dtsi is useless and will produce the following
warning when compiled with W=1:

Node /memory has a reg or ranges property, but no unit name

Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>