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/Makefile b/drivers/i2c/muxes/Makefile
index 612cc27..47c1240 100644
--- a/drivers/i2c/muxes/Makefile
+++ b/drivers/i2c/muxes/Makefile
@@ -4,4 +4,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 obj-$(CONFIG_I2C_ARB_GPIO_CHALLENGE) += i2c-arb-gpio-challenge.o
-obj-$(CONFIG_I2C_MUX) += i2c-mux-uclass.o
+obj-$(CONFIG_$(SPL_)I2C_MUX) += i2c-mux-uclass.o