ARC: [plat-axs10x] add Ethernet PHY description in .dts

Commit e34d6569 ("stmmac: create of compatible mdio bus for stmmac
driver") broke DW GMAC functionality on ARC AXS10x boards:

That's what happens on eth0 up:
  --------------------------->8------------------------
| libphy: PHY stmmac-0:ffffffff not found
| eth0: Could not attach to PHY
| stmmac_open: Cannot attach to PHY (error: -19)
  --------------------------->8------------------------

Simplest solution is to add PHY description in board's .dts.
And so we do here.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Phil Reid <preid@electromag.com.au>
Cc: David S. Miller <davem@davemloft.net>
Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: stable@vger.kernel.org # 4.5
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>