arm/acpi: add placeholder for efi and acpi load address

arm/acpi: add placeholder for efi and acpi load address

We will create EFI table, memory description table and some of acpi
tables and we're going to map them to kinfo->gnttab_start of Dom0.
Add placeholder for the starting address for loading in DOM0 and the
size of new added tables. Also add a placeholder to store the new
created tables.

Signed-off-by: Parth Dixit <parth.dixit@linaro.org>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>