GitLab

x8 is not a callee-saved register and can be corrupted.
Use x19 instead to be AAPCS-compliant.

Fixes ARM-software/tf-issues#478

Change-Id: Ib4f114c36f4c11351ae856f953c45dca92b27c3b
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>