docs: support solution on ampere altra only

The solution use cases are validated on Ampere Altra and Ubuntu 20.04
platform, so limit the supported platforms to Ampere Altra only.

Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Change-Id: Ib88ce3a773911617d7c8e85dbef84c1e35dcd5ab