spi: sam: Specify pinmux for chipselect via Kconfig

Move to using Kconfig (like other Atmel SAM drivers do) to specify the
pinmux setting for SAMe70 SoCs.  Updated the sam_e70_xplained board to
set the default in Kconfig.defconfig instead of via board.h

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