Actually changing defconfig manually or using menuconfig will lets you select I2CGPIO and I2CGPIO Fault Injector so the kernel knows about the driver but does not create any bus instance or inform the kernel which pins to use.
You should create a device Tree to tells the kernel “make a software I2C bus using these GPIO pins.”The i2c-gpio driver looks for a specific node in the device tree to understand which GPIO pins to use for the I²C bus. Without this node, the driver has no bus to attach to.
This node must be added to your AST2600's specific .dts file or one of the .dtsi files it includes.