include/device: Split i2c.h into three

Split `i2c.h` into three pieces to ease reuse of the generic defi-
nitions. No code is changed.

* `i2c.h`        - keeps the generic definitions
* `i2c_simple.h` - holds the current, limited to one controller driver
                   per board, devicetree independent I2C interface
* `i2c_bus.h`    - will become the devicetree compatible interface for
                   native I2C (e.g. non-SMBus) controllers

Change-Id: I382d45c70f9314588663e1284f264f877469c74d
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/20845
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/drivers/intel/mipi_camera/camera.c

@@ -17,7 +17,7 @@
 #include <arch/acpi_device.h>
 #include <arch/acpigen.h>
 #include <console/console.h>
-#include <device/i2c.h>
+#include <device/i2c_simple.h>
 #include <device/device.h>
 #include <device/path.h>
 #include <string.h>