arc: Add "model" properly in device tree description of all boards

arc: Add "model" properly in device tree description of all boards

As it was discussed quite some time ago (see
https://lkml.org/lkml/2015/11/5/862) it's a good practice to add
"model" property in .dts. Moreover as per ePAPR "model" property is
required and should look like "manufacturer,model" so we do here.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Jonas Gorski <jonas.gorski@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rob Herring <robh@kernel.org>
Cc: Christian Ruppert <christian.ruppert@alitech.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>