arm64/mm: Add FEAT_LPA2 specific VTCR_EL2.DS field

As per ARM ARM (0487G.A) VTCR_EL2.DS fields controls whether 52 bit IPA and
output physical address get supported on 4K and 16K page size configuration
when FEAT_LPA2 is known to have been implemented. This adds VTCR_DS field
definition which would be used when FEAT_LPA2 gets enabled in Stage-2.

Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>