mfd: cros ec: spi: Depend on OF

The driver is not used on any non-DT platform, so it can depend on the
OF Kconfig symbol to make that explicit. This is in preparation of a
subsequent patch which parses some parameters from the device tree and
would otherwise have to conditionalize that code.
Signed-off-by: NThierry Reding <treding@nvidia.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

drivers/mfd/Kconfig

@@ -80,7 +80,7 @@ config MFD_CROS_EC_I2C
 
 config MFD_CROS_EC_SPI
 	tristate "ChromeOS Embedded Controller (SPI)"
-	depends on MFD_CROS_EC && SPI
+	depends on MFD_CROS_EC && SPI && OF
 
 	---help---
 	  If you say Y here, you get support for talking to the ChromeOS EC