ARM: keystone: Add minimal TI Keystone platform support

Texas Instruments Keystone family of multi-core devices are
based on ARM Cortex A15. Patch adds basic definitions for a
new Keystone sub-architecture in ARM.

The TCI66xxK2H Communications Infrastructure Keystone SoCs
are member of the C66x family based on TI's new KeyStone 2
multi-core SoC Architecture designed specifically for high
performance wireless and networking infrastructure applications.
The SOCs contains many subsystems like Cortex A15 ARM CorePacs,
C66XX DSP CorePacs, MSMC memory controller, Tera Net bus,
IP Network, Navigator, Hyperlink, 1G/10G Ethernet, Radio layers
and queue based communication systems.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: arm@kernel.org

Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>