kvm: arm64: Clean up VTCR_EL2 initialisation

Use the new helper for converting the parange to the physical shift.
Also, add the missing definitions for the VTCR_EL2 register fields
and use them instead of hard coding numbers.

Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Christoffer Dall <cdall@kernel.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>