arm64: KVM: Fix outdated comment about VTCR_EL2.PS

Commit 87366d8cf7b3 ("arm64: Add boot time configuration of
Intermediate Physical Address size") removed the hardcoded setting
of VTCR_EL2.PS to use ID_AA64MMFR0_EL1.PARange instead, but didn't
remove the (now rather misleading) comment.

Fix the comments to match reality (at least for the next few minutes).

Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>