spi: Remove obsolete spi_base_setup_slave_fdt

0efc0249 ("spi_flash: Add spi_flash_probe_fdt() to locate SPI by FDT
node") added a helper function spi_base_setup_slave_fdt to to set up a
SPI slave from a given FDT blob. The only user was the exynos SPI
driver.

But commit 73186c94

 ("dm: exynos: Convert SPI to driver model") removed
the use of this function, hence rendering it obsolete.

Remove this function, as well as the CONFIG_OF_SPI option, which guarded
only this function.

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Signed-off-by: Mario Six <mario.six@gdsys.cc>