mx6: add common SPL configuration

mx6: add common SPL configuration

Add a common header which can hopefully be shared amon imx6 SPL users

Cc: Stefan Roese <[email protected]>
Cc: Otavio Salvador <[email protected]>
Cc: Andy Ng <[email protected]>
Cc: Eric Nelson <[email protected]>
Cc: Tapani Utriainen <[email protected]>
Cc: Tom Rini <[email protected]>
Signed-off-by: Tim Harvey <[email protected]>
Acked-by: Stefano Babic <[email protected]>
Acked-by: Nikita Kiryanov <[email protected]>