refactor(el3_runtime): introduce save_x30 macro

Most of the macros/routine in vector entry need a free scratch register.
Introduce a macro "save_x30" and call it right at the begining of vector
entries where x30 is used. It is more exlicit and less error prone

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I617f3d41a120739e5e3fe1c421c79ceb70c1188e
3 files changed