iommu: Introduce DOMAIN_ATTR_SECURE_VMID

ARM iommus may support 2 stages of translation. In this case the base
page table stage1 addresses are actually intermediate physical address
rather than physical addresses and are subject to stage2 translation.
Add a new attribute to specify which stage2 table the stage1 base page
table needs to be mapped into.

Change-Id: I7cd008edbd3de31a9d2826e52e2e0d5b971275a9
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>