plat/qemu: Use RNDR in stack protector

When getting a stack protector canary value, check
if cpu supports FEAT_RNG and use that. Fallback to
old method of using a (hardcoded value ^ timer).

Signed-off-by: Tomas Pilar <tomas@nuviainc.com>
Change-Id: I8181acf8e31661d4cc82bc3a4078f8751909e725
1 file changed