[TF-A] Query BL31 version from NWd

Hi,

Happy new year!

For NVIDIA Tegra platforms, we need the capability to query the BL31 version (e.g. 2.7, 2.8, 2.8.x) from the NWd at runtime. I could not find an FID that returns this value.

I propose we introduce a new runtime service in bl31 that returns the version_string to the NWd to support this requirement.

Thoughts?

-Varun