arm64: dts: k3-am6: Add "socionext,synquacer-pre-its" property to gic_its

GIC_ITS used in AM654 platform has the same configuration as that of
GIC_ITS used in Socionext SoCs. Add "socionext,synquacer-pre-its"
property to get PCI MSI working.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>