arm: Invalidate icache on prefetch abort outside of user mapping on Cortex-A15

In order to prevent aliasing attacks on the branch predictor,
invalidate the icache on Cortex-A15, which has the side effect
of invalidating the BTB. This requires ACTLR[0] to be set to 1
(secure operation).

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>