arm: imx: hab: Add IVT header definitions

The various i.MX BootROMs containing the High Assurance Boot (HAB) block
rely on a data structure called the Image Vector Table (IVT) to describe to
the BootROM where to locate various data-structures used by HAB during
authentication.

This patch adds a definition of the IVT header for use in later patches,
where we will break the current incorrect dependence on fixed offsets in
favour of an IVT described parsing of incoming binaries.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Sven Ebenfeld <sven.ebenfeld@gmail.com>
Cc: George McCollister <george.mccollister@gmail.com>
Cc: Breno Matheus Lima <brenomatheus@gmail.com>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>