arm64, thunder: Add Kconfig option for Cavium Thunder SoC Family

This introduces ARCH_THUNDER to enable soc specific drivers and dtb
files.

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>