Documentation: arm64: Document ARM Neoverse-N1 erratum #1688567

According to erratum #1688567, a SPE buffer write that results in an Access
flag fault or Permission fault at stage 2 is reported with an unsupported
PMBSR_EL1.FSC code.

KVM avoids SPE stage 2 faults altogether by requiring userspace to lock the
guest memory in RAM and pre-mapping it in stage 2 before the VM is started.
As a result, KVM is not impacted by this erratum.