ARM: spectre-v2: add Cortex A8 and A15 validation of the IBE bit

When the branch predictor hardening is enabled, firmware must have set
the IBE bit in the auxiliary control register.  If this bit has not
been set, the Spectre workarounds will not be functional.

Add validation that this bit is set, and print a warning at alert level
if this is not the case.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Boot-tested-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>