arm64: dts: move from ARCH_VULCAN to ARCH_THUNDER2

Move and update device tree files as part of transition from Broadcom
Vulcan to Cavium ThunderX2.

The changes are to:
 * rename dts/broadcom/vulcan.dtsi to cavium/thunder2-99xx.dtsi,
   update cpu cores to be "cavium,thunder2", and update SoC to be
   "cavium,thunderx2-cn9900"
 * move SoC dts/broadcom/vulcan-eval.dtsi to cavium/thunder2-99xx.dtsi
   and update board name string
 * Update dts/broadcom/Makefile not to build vulcan dtbs
 * Update dts/cavium/Makefile to build thunder2 dtbs

No changes to the dts contents except the updated "compatible" and
"model" properties.

Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>