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/db-mv784mp-gp_defconfig b/configs/db-mv784mp-gp_defconfig
index 4c3240a..a9e7f04 100644
--- a/configs/db-mv784mp-gp_defconfig
+++ b/configs/db-mv784mp-gp_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_MVEBU=y
 CONFIG_TARGET_DB_MV784MP_GP=y
+CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="armada-xp-gp"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_BOOTDELAY=3