SECURE BOOT: change prototype of fsl_secboot_validate function

The prototype and defination of function fsl_secboot_validate
has been changed to support calling this function from another
function within u-boot.
Only two aruments needed:
1) header address - Mandatory
2) SHA256 string - optional

Signed-off-by: Saksham Jain <saksham@freescale.com>
Signed-off-by: Aneesh Bansal <aneesh.bansal@freescale.com>
Acked-by: Ruchika Gupta <ruchika.gupta@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>