docs: update dependent utilities

The git-repo has to be installed manually because repo installed via
apt-get has issues on Ubuntu.
Updated the utilities required if running solution on a fresh installed
environment.

Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Change-Id: I9591181a77cea5239091835da61295fb4269fef3