ARM: add comments to the early page table remap code

Add further comments to the early page table remap code to explain what
the code is doing, why it is doing it, but more importantly to explain
that the code is not architecturally compliant and is squarely in
"UNPREDICTABLE" behaviour territory.

Add a warning and tainting of the kernel too.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>