mx6cuboxi: Differentiate Cubox-i and Hummingboard

Introduce is_hummingboard() function that reads GPIOs that can distinguish
between Cubox-i and Hummingboard.

Print the board name accordingly.

Based on a patch from Rabeeh Khoury.

Signed-off-by: Rabeeh Khoury <rabeeh@solid-run.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Tom Rini <trini@konsulko.com>