arm64: Implement branch predictor hardening for Falkor

Falkor is susceptible to branch predictor aliasing and can
theoretically be attacked by malicious code. This patch
implements a mitigation for these attacks, preventing any
malicious entries from affecting other victim contexts.

Change-Id: I535d423c2cefaf93627267b867bf0846e502d4c1
Signed-off-by: Shanker Donthineni <shankerd@codeaurora.org>
[will: fix label name when !CONFIG_KVM and remove references to MIDR_FALKOR]
Signed-off-by: Will Deacon <will.deacon@arm.com>
[sramana@codeaurora.org: Use only the link stack sanitization routines,
and leave Falkor related BP hardening code]
Signed-off-by: Srinivas Ramana <sramana@codeaurora.org>