drivers: spi: spi_sam: rework device tree support

Reworked spi_sam driver to utilize new DT_INST macros as part of
this rework we also now get pin ctrl/mux configuration information
from the device tree instead of via Kconfig and defines in soc_pinmap.h

We remove defines from dts_fixup.h and soc_pinmap.h and associated
Kconfig symbols that are no longer needed due to getting all that
information from devicetree.

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