dm: i2c: Allow muxes to be enabled for SPL separately

Since I2C muxes are seldom needed in SPL, and the code for this increases
the size somewhat, add a separate option to enable I2C muxes for SPL.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/i2c/muxes/Kconfig b/drivers/i2c/muxes/Kconfig
index 913093d..f959d9d 100644
--- a/drivers/i2c/muxes/Kconfig
+++ b/drivers/i2c/muxes/Kconfig
@@ -7,6 +7,15 @@
 	  bus select is handled automatically when that bus is accessed,
 	  using a suitable I2C MUX driver.
 
+config SPL_I2C_MUX
+	bool "Support I2C multiplexers on SPL"
+	depends on I2C_MUX
+	help
+	  This enables I2C buses to be multiplexed, so that you can select
+	  one of several buses using some sort of control mechanism. The
+	  bus select is handled automatically when that bus is accessed,
+	  using a suitable I2C MUX driver.
+
 config I2C_ARB_GPIO_CHALLENGE
 	bool "GPIO-based I2C arbitration"
 	depends on I2C_MUX