[ARM] generic: Add DTS file for two VExpress-A15 guests on VExpress-A15 host

There is lot of interest in having more than one instance of VExpress-A15
guests hence this patch adds an example DTS for having two pre-configured
VExpress-A15 guests on VExpress-A15 host.

Signed-off-by: Anup Patel <anup@brainfault.org>