ARM: Introduce ability to enable ACR::IBE on Cortex-A8 for CVE-2017-5715

ARM: Introduce ability to enable ACR::IBE on Cortex-A8 for CVE-2017-5715

As recommended by Arm in [1], IBE[2] has to be enabled unconditionally
for BPIALL to be functional on Cortex-A8 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.ddi0344k/Bgbffjhh.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]>