KVM: Documentation: Correct the VGIC V2 CPU interface addr space size

In arch/arm64/include/uapi/asm/kvm.h, we have

  #define KVM_VGIC_V2_CPU_SIZE		0x2000

So the CPU interface address space should cover 8 KByte not 4 KByte.

Signed-off-by: Changyuan Lyu <changyuanl@google.com>
Link: https://lore.kernel.org/r/20240623164542.2999626-3-changyuanl@google.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>