ARM: keystone: add documentation for SoCs and EVMs

Currently there is no general documentation on Keystone SoCs in the
Linux Documentation folder of the source tree. This patch adds some
essential documentation with links to help users of Keystone Linux and
also provide links to existing documents where necessary.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>