i2c: add CONFIG_DM_I2C to Kconfig

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>

drivers/i2c/Kconfig

+config DM_I2C
+	bool "Enable Driver Model for I2C drivers"
+	depends on DM
+	help
+	  If you want to use driver model for I2C drivers, say Y.
+	  To use legacy I2C drivers, say N.