Add RD-Aspen platform documentation

Add a documentation page for RD-Aspen which includes a basic
overview of the platform, the secure services used and a link
to the official documentation.

Change-Id: Ic2eeb751dd266c2155022c671aa58947ee39b7f7
Signed-off-by: Meet Patel <meet.patel2@arm.com>
2 files changed