media: atmel: atmel-isc: create callback for GAM submodule product specific

The GAM submodule is a part of the atmel-isc pipeline, and stands for
Gamma Correction. It is used to apply the gamma curve to the incoming pixels.
Create a product specific callback for initializing the GAM submodule
of the pipeline.
For sama5d2 product, there is no special configuration at this moment,
thus this function is a noop.

[hverkuil: made isc_sama5d2_config_gam static]

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>