arm: initial machine port for artpec-6 SoC

Basic machine port for the Artpec-6 SoC from Axis
Communications.

Signed-off-by: Lars Persson <larper@axis.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>