spi: altera: Make SPI_ALTERA_CORE invisible

The SPI_ALTERA_CORE config symbol controls compilation of the Altera SPI
Controller core code.  It is already selected by all of its users, so
there is no reason to make it visible, unless compile-testing.

Fixes: b0c3d9354d ("spi: altera: separate core code from platform code")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/f0cb8e66baba4506db6f42fca74dc51b76883507.1619534253.git.geert+renesas@glider.be
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/Kconfig

@@ -65,7 +65,7 @@ config SPI_ALTERA
 	  This is the driver for the Altera SPI Controller.
 
 config SPI_ALTERA_CORE
-	tristate "Altera SPI Controller core code"
+	tristate "Altera SPI Controller core code" if COMPILE_TEST
 	select REGMAP
 	help
 	  "The core code for the Altera SPI Controller"