arm: imx: fix signature_block_hdr struct fields order

arm: imx: fix signature_block_hdr struct fields order

According to the documentation (for example NXP's AN13994 on encrypted
boot on AHAB-enabled devices), the format of the signature block is:

  +--------------+--------------+--------------+-------------+
  | Tag          | Length - msb | Length - lsb | Version     |
  +--------------+--------------+--------------+-------------+
  | SRK Table offset            | Certificate offset         |
  +-----------------------------+----------------------------+
  | Blob offset                 | Signature offset           |
  +-----------------------------+----------------------------+

There is no runtime error in the current u-boot code. The only user of
struct signature_block_hdr is the "get_container_size" function in the
"arch/arm/mach-imx/image-container.c" file, and it's only using the very
first fields of the struct (which are in the correct position) and thus
there is no runtime failure.

On the other hand, extending the code to get the data encryption key
blob offset on the signature header gives a wrong value as the field is
in the wrong order.

Signed-off-by: Javier Viguera <javier.viguera@digi.com>