ARM: dts: Fix memory node in skeleton64.dtsi

Update the reg property of the memory node in
skeleton64.dtsi to reflect the fact that the root node uses
address-cells=2 and size-cells=2.

Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>