OvmfPkg/IoMmuDxe: rename DeviceAddress to PlainTextAddress in MAP_INFO

OvmfPkg/IoMmuDxe: rename DeviceAddress to PlainTextAddress in MAP_INFO

In this particular IOMMU driver, "DeviceAddress" is just as accessible to
the CPU as "HostAddress", the difference is that the area pointed-to by
the former is plain-text and accessible to the hypervisor. Rename
"DeviceAddress" to "PlainTextAddress" in MAP_INFO.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Suggested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>