arm64: dts: rockchip: add missing #msi-cells to rk3399

The rk3399 gic-its was missing the #msi-cells property as found by
dt-schema checks, so add it.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20190917083625.25818-1-heiko@sntech.de