sky2: allow mac to come from dt

The driver reads the mac address from the device registers which would
need to have been programmed by the bootloader.  This patch adds
the ability to pull the mac from devicetree via the pci device dt node.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Cc: netdev@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>

Changes since v2:
 - eliminated use of stack tmpaddr per feedback

Changes since v1:
 - simplified based on feedback
 - fixed formatting
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>