arm64: Enable and document ARM errata 1319367 and 1319537

Now that everything is in place, let's get the ball rolling
by allowing the corresponding config option to be selected.
Also add the required information to silicon_errata.rst.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>