KVM: arm64: Update help text

arm64 KVM supports 16k pages since 02e0b760


("arm64: kvm: Add support for 16K pages"), so update the Kconfig help
text accordingly.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200505154520.194120-3-tabba@google.com