spi: dw: remove unused dw_spi_chip handling

The path of code using this struct is unused since there is no more user
of this. Remove code and struct definition.

Signed-off-by: Clement Leger <cleger@kalray.eu>
Link: https://lore.kernel.org/r/20200416110916.22633-1-cleger@kalray.eu


Signed-off-by: Mark Brown <broonie@kernel.org>