ARM: Introduce ability to enable invalidate of BTB with ICIALLU on Cortex-A15 for CVE-2017-5715

As recommended by Arm in [1], ACTLR[0] (Enable invalidates of BTB)
needs to be set[2] for BTB to be invalidated on ICIALLU. This needs to
be done unconditionally for Cortex-A15 processors. Provide a config
option for platforms to enable this option based on impact analysis
for products.

NOTE: This patch in itself is NOT the final solution, this requires:
a) Implementation of v7_arch_cp15_set_acr on SoCs which may not
   provide direct access to ACR register.
b) Operating Systems such as Linux to provide adequate workaround in the
   right locations.
c) This workaround applies to only the boot processor. It is important
   to apply workaround as necessary (context-save-restore) around low
   power context loss OR additional processors as necessary in either
   firmware support OR elsewhere in OS.

[1] https://developer.arm.com/support/security-update
[2] http://infocenter.arm.com/help/topic/com.arm.doc.ddi0438c/BABGHIBG.html

Cc: Marc Zyngier <[email protected]>
Cc: Russell King <[email protected]>
Cc: Tony Lindgren <[email protected]>
Cc: Robin Murphy <[email protected]>
Cc: Florian Fainelli <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Christoffer Dall <[email protected]>
Cc: Andre Przywara <[email protected]>
Cc: Ard Biesheuvel <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Michael Nazzareno Trimarchi <[email protected]>

Signed-off-by: Nishanth Menon <[email protected]>
Tested-by: Fabio Estevam <[email protected]>

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