SECURE BOOT: change prototype of fsl_secboot_validate function

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 <[email protected]>
Signed-off-by: Aneesh Bansal <[email protected]>
Acked-by: Ruchika Gupta <[email protected]>
Reviewed-by: York Sun <[email protected]>