drivers: ethernet: mcux: rework how unique mac is determined

Instead of having a Kconfig property, if there is no local-mac-address
property in the devicetree than we'll generate a unique MAC address
based on unique ID registers on the SoC.

We remove the local-mac-address properties in the SoC dtsi files to
match the default behavior that existed before (ie, unique MAC address)

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>