arm: Add BTB invalidation on switch_mm for Cortex-A73

Invalidate the BTB when switching from one user context to
another, to avoid aliasing attacks against the branch predictor.

Change-Id: I175931afda9b81f961e6c0831da299d2f79aa175
Signed-off-by: Teng Fei Fan <tengfei@codeaurora.org>