imx6: add open part for HAB signing

This directory contain the open part of Apertis super root keys used for
signing U-Boot and FIT kernel image for i.MX-based boards (SabreLite).

The private keys and password are set in CI/CD as a secret, however all bits are available
in [open repository](https://gitlab.apertis.org/infrastructure/apertis-imx-srk

).

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>