hikey960: Fix 404 links

The build instructions for the hikey960 had some broken links. Update
the links to use new vendor URLs. Also change build instructions to
reference a different file name.

Signed-off-by: Dylan Corrales <deathcamel58@gmail.com>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>