On Qualcomm platforms, the TZ may already have certain memory regions
under protection by the time U-Boot starts. There is a rare case on some
platforms where the prefetcher might speculatively access one of these
regions resulting in a board crash (TZ traps and then resets the board).

We shouldn't be accessing these regions from within U-Boot anyway, so
let's mark them all with PTE_TYPE_FAULT to prevent any speculative
access and correctly trap in EL1 rather than EL3.

This is quite costly with caches off (takes ~2 seconds on SDM845 vs 35ms
with caches on). So to minimise the impact this is only enabled on
QCS404 for now (where the issue is known to occur).

In the future, we should try to find a more efficient way to handle
this, perhaps by turning on the MMU in stages.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>