KVM: arm64: Fix comment for KVM_PHYS_SHIFT

Since Suzuki K Poulose's work on Dynamic IPA support, KVM_PHYS_SHIFT will
be used only when machine_type's bits[7:0] equal to 0 (by default). Thus
the outdated comment should be fixed.
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm64/include/asm/kvm_mmu.h

@@ -138,7 +138,8 @@ static inline unsigned long __kern_hyp_va(unsigned long v)
 	})
 
 /*
- * We currently only support a 40bit IPA.
+ * We currently support using a VM-specified IPA size. For backward
+ * compatibility, the default IPA size is fixed to 40bits.
  */
 #define KVM_PHYS_SHIFT	(40)