media: atmel: atmel-isc: specialize gamma table into product specific

Separate the gamma table from the isc base file into the specific sama5d2
product file.
Add a pointer to the gamma table and entries count inside the platform
driver specific struct.

[hverkuil: made isc_sama5d2_gamma_table 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>