dts: npcx7m6fb: define the port as i2c bus rather than the controller

The npcx7m6fb dts defines i2c controller and ports, where the controller
is a property in the port node definition and the device nodes should be
defined as subnodes of the port.

This changes the binding definitions to reflect that, defining the port
as an i2c-controller so that in can inherit sub-nodes for i2c devices,
and the controller as a generic node.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>