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/Makefile b/drivers/i2c/Makefile
index 811ad9b..bbbc0dc 100644
--- a/drivers/i2c/Makefile
+++ b/drivers/i2c/Makefile
@@ -7,8 +7,8 @@
 obj-$(CONFIG_DM_I2C) += i2c-uclass.o
 obj-$(CONFIG_DM_I2C_COMPAT) += i2c-uclass-compat.o
 obj-$(CONFIG_DM_I2C_GPIO) += i2c-gpio.o
-obj-$(CONFIG_I2C_CROS_EC_TUNNEL) += cros_ec_tunnel.o
-obj-$(CONFIG_I2C_CROS_EC_LDO) += cros_ec_ldo.o
+obj-$(CONFIG_$(SPL_)I2C_CROS_EC_TUNNEL) += cros_ec_tunnel.o
+obj-$(CONFIG_$(SPL_)I2C_CROS_EC_LDO) += cros_ec_ldo.o
 
 obj-$(CONFIG_SYS_I2C_ADI) += adi_i2c.o
 obj-$(CONFIG_I2C_MV) += mv_i2c.o