KVM: arm64: Document the KVM/arm64-specific calls in hypercalls.rst

KVM: arm64: Document the KVM/arm64-specific calls in hypercalls.rst

KVM/arm64 makes use of the SMCCC "Vendor Specific Hypervisor Service
Call Range" to expose KVM-specific hypercalls to guests in a
discoverable and extensible fashion.

Document the existence of this interface and the discovery hypercall.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
Acked-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240423150538.2103045-28-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>