arm64, thunder: Add initial dts for Cavium Thunder SoC

Add initial device tree nodes for Cavium Thunder SoCs with support of
48 cores and gicv3. The dtsi file requires further changes, esp. for
pci, gicv3-its and smmu. This changes will be added later together
with the device drivers.

Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
Signed-off-by: Robert Richter <rrichter@cavium.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>