sign/imx6: Add private keys as well from the Apertis HABv4 repository

Our keys are not meant to be actually secret since they are only for demonstrational purposes.

The goal is to show how product teams can use their own keys rathet than providing any kind of protection for the test devices.

Given that once a HAB key is flashed on the i.MX6 boards it can't be changed, it is desirable to use a widely available key for testing to avoid bricking the device.

All the keys come from the Apertis HABv4 keys repository: https://gitlab.apertis.org/infrastructure/apertis-imx-srk/