dts: edtlib: Turn global spi_dev_cs_gpio() into Node.spi_cs_gpio

spi_dev_cs_gpio() takes a Node and returns the chip select GPIO for it.
Having that information available directly from Node is neater, so turn
it into a Node.spi_cs_gpio property instead.

That gets rid of the only public global function in edtlib, which might
make the API design clearer too.

Tested with the sensortile_box board, which uses SPI chip select.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>