netspace_v2: fix SDRAM configuration

Network Space (Max) v2 and Internet Space v2 boards have different
SDRAM configuration and size: respectively 256MB (DDR2 1Gb, 128Mbx8
organisation) and 128MB (DDR2 512Mb, 64Mbx8 organisation). To handle
the differences, this patch add a dedicated kwimage.cfg file for
Internet Space v2.

Additionnaly the number of SDRAM banks is fixed.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>