Convert CONFIG_SPL_I2C_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/etamin_defconfig b/configs/etamin_defconfig
index 9d0470c..54eb720 100644
--- a/configs/etamin_defconfig
+++ b/configs/etamin_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_SPL_FAT_SUPPORT=y
+CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="am335x-draco"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y