[Hafnium] prebuilts gn tool update

Hi,

Fyi we merged this gn tool prebuilt version update https://review.trustedfirmware.org/q/topic:%22od%252Fgn-update-1901%22+(stat...)

Essentially more recent sources were used to build the x86 host version (as the tool hasn't been updated since 2018), and this aligns to the version used on aarch64 host.

'make format' uses gn to format .gn build files, and as a consequence new formatting rules apply with the new tool version. The side effect is that changes already submitted in the hafnium dashboards would have to be rebased, and run make format again to pass the CI.

Regards, Olivier.