infra: document support for booting on AArch64 hosts

The RD-N2 and RD-N2-Cfg1 platform software stack can be booted on a FVP
built for a AArch64 host machine. Although the procedure to boot on a
AArch64 host is same as that on a x86 host, the build of the software
stack has to be performed on a x86 host. Add a document to explain
these steps.

Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>
Change-Id: Icf6a2cd1af54b22145a31ad95e1b484e8bf44f7c