metag: minimal TZ1090 (Comet) SoC infrastructure

Add really minimal support for Toumaz Xenif TZ1090 SoC (A.K.A. Comet).
This consists of minimal build infrastructure, device tree files, and a
defconfig based on meta2_defconfig.

This SoC contains a 2-threaded HTP (Meta 2) as the main application
processor, and is found in a number of development boards and digital
radios, such as the Minimorph Development Platform.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Rob Landley <rob@landley.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-doc@vger.kernel.org
Cc: devicetree-discuss@lists.ozlabs.org