xtensa: add xtavnet platform (based on Avnet LX60 and LX200 boards)

Add support for the Tensilica-configured Avnet emulation boards.
These include the LX60 (XT-AV60) and LX200 (XT-AV200) boards.
Note that because these are FPGA based, any number of different processor
variants can run on them.  The larger LX200 can also support SMP.

Documentation for these boards is available from Tensilica
(e.g. in their free eval downloads) in documents titled,
"Tensilica Avnet LX60 (XT-AV60) Board User's Guide", and
"Tensilica Avnet LX200 (XT-AV200) Board User's Guide".

Derived from the arch/xtensa/platforms/lx60 platform code in
git://git.linux-xtensa.org/git/kernel/xtensa-2.6.29-smp.git,

Signed-off-by: Dan Nicolaescu <dann@xtensa-linux.org>
Signed-off-by: Pete Delaney <piet@tensilica.com>
Signed-off-by: Marc Gauthier <marc@tensilica.com>
Signed-off-by: Chris Zankel <chris@zankel.net>