sandbox: Move CONFIG_SANDBOX_SPI to Kconfig

Move this over to Kconfig and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 56e7deb..058949a 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -20,3 +20,4 @@
 CONFIG_SANDBOX_GPIO=y
 CONFIG_SYS_VSNPRINTF=y
 CONFIG_SYS_I2C_SANDBOX=y
+CONFIG_SANDBOX_SPI=y