arm64, thunder: Document devicetree bindings for Cavium Thunder SoC

This patch adds documentation for the devicetree bindings used by the
DT files of Cavium Thunder SoC platforms.

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>