mx6cuboxi: fix 4GB ddr memory detection

The soms with 4GB ddr have a rowaddr of 16 not 15, this allows
the detection mechanism to properly identify them as 4GB.
However these soms can be populated with whatever amount of
memory the customer requests therefor we need a ram stride test.
We can not use the get_ram_size() function because not all 4GB's
of DDR is addressable on a 32-bit architecture.  Therefore instead
we use a memory stride of 128MB's and look for the address that
the memory wraps.  This function is used for all som types to
catch most memory configurations.

This is a revised version of Rabeeh Khoury's original code.

Signed-off-by: Jon Nettleton <jon@solid-run.com>
Signed-off-by: Rabeeh Khoury <rabeeh@solid-run.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>