drivers: ethernet: sam_gmac: rework pin config

Reworked sam_gmac driver to get pin ctrl/mux configuration information
from the device tree instead of via Kconfig and defines in soc_pinmap.h

We remove defines from soc_pinmap.h that are no longer needed due to
getting all that information from devicetree.

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