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.comSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

Documentation/virt/kvm/devices/arm-vgic.rst

@@ -31,7 +31,7 @@ Groups:
     KVM_VGIC_V2_ADDR_TYPE_CPU (rw, 64-bit)
       Base address in the guest physical address space of the GIC virtual cpu
       interface register mappings. Only valid for KVM_DEV_TYPE_ARM_VGIC_V2.
-      This address needs to be 4K aligned and the region covers 4 KByte.
+      This address needs to be 4K aligned and the region covers 8 KByte.
 
   Errors: