arm64: QEMU virt + TF-A test environment, kerf support and docs #20

Part of the arm64 port.

A reproducible environment that exercises every arm64 mechanism except real SMMU behaviour:

• QEMU -M virt,secure=on,gic-version=3,its=on -cpu max with TF-A (BL1/BL2/BL31 with SDEI and PSCI) booting the host at EL2, several vCPUs, one or two virtio-pci devices to assign.
• Host kernel config: CONFIG_MULTIKERNEL=y, ARM_SDE_INTERFACE, ARM64_PSEUDO_NMI, KEXEC_FILE.
• kerf (tools/multikernel/ or the multikernel/kerf repo) support for arm64: Image instead of vmlinux/bzImage, DTB handling, MPIDR-based CPU names.
• Scripted smoke test: spawn on 2 CPUs, message round-trip over the ring, graceful halt (all CPUs OFF), forced halt, re-spawn, CPU hot-add to a running spawn.
• Document under Documentation/multikernel/ (arm64 section in usage.rst).

Real hardware target after QEMU: an SBSA server (Neoverse-based, PSCI + GICv3 + ITS + ECAM + TF-A/SDEI).