spi: Kconfig: Remove old comments now that SPI slave is supported

Since commit 6c364062 ("spi: core: Add support for registering SPI
slave controllers") SPI slave is also supported, so remove the old
comments that say SPI slave is unsupported.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>