mx6cuboxi: consolidate board detection and add som revision checking

mx6cuboxi: consolidate board detection and add som revision checking

In order to properly detect the board the checks need to be done
in a specific order.  Move these tests back into a single enum
function that will always return the proper the board it is checking.

This also adds the best test we have for detecting the rev 1.5 som,
and it simplifies the device-tree filename building.

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