ARM: dts: r8a7740: Remove skeleton.dtsi inclusion

As of commit 9c0da3cc ("ARM: dts: explicitly mark skeleton.dtsi
as deprecated"), including skeleton.dtsi is deprecated.

This fixes the following warning with W=1:

    Warning (unit_address_vs_reg): Node /memory has a reg or ranges property, but no unit name

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>