ARC: Add model property to boards .dts

1. This way we sync with Linux kernel where we have model
   set for all ARC boards for quite some time, see [1]

2. Once we enable DISPLAY_BOARDINFO for ARC this info will
   be printed on boot givin some extra data-point about the board

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=618a9cd06dd471ac232f5b27325b24d26eba5571



Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>