docs/aemfvp-a: add user guide for aemfvp-a

The aemfvp-a documents explain the steps to download, build the software
stacks and run these software stacks on the Armv-A Base AEM FVP.

Signed-off-by: Selvarasu Ganesan <selvarasu.ganesan@arm.com>
Change-Id: I81f548886901c2b03074283df1ec7d7558d26166