juno: Invalidate all caches before warm reset to AArch32 state.

On Juno AArch32, the L2 cache may contain garbage after the warm reset
from AArch64 to AArch32.  This is all fine until the MMU is configured
and the data caches enabled.  To avoid fetching stale data from the L2
unified cache, invalidate it before the warm reset to AArch32 state.

Change-Id: I7d27e810692c02c3e83c9f31de67f6bae59a960a
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>