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/maxbcm_defconfig b/configs/maxbcm_defconfig
index 0e0999c..e65226c 100644
--- a/configs/maxbcm_defconfig
+++ b/configs/maxbcm_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_MVEBU=y
 CONFIG_TARGET_MAXBCM=y
+CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="armada-xp-maxbcm"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y