Move up the version numbers in the user guide of:

 * DS-5 (to v5.22)
 * Base FVP (to 7.0)
 * Foundation FVP (to 9.4)
 * Linaro release (to 15.10)

Note that, starting from Linaro release 15.10, the related release
instructions have migrated from http://releases.linaro.org to the
ARM Connected Community website. The URLs in the User Guide have
been updated accordingly.

The 'libssl-dev' package has been removed from the list of
prerequisite tools, as it is already referenced on the ARM Connected
Community page. Also, the 'device-tree-compiler' package has been
marked as an optional dependency, since the Trusted Firmware
repository provides the prebuilt DTB files. Hence, this tool is
needed only when the user wants to rebuild the DTS files.

Change-Id: I4a172ece60bf90437131c6b96e73a9f1e9b40117