Migrate to Linaro release 17.04

ARM TF has been tested against Linaro Release 17.04 - the Linaro
binaries have been update and also the version of the compiler.

Linaro binaries: 17.01 --> 17.04
AArch64 & AArch32 compilers: 5.3-2015.05 (gcc 5.3) -> 6.2-2016.11 (gcc 6.2)

This patch updates the User Guide is to state that Linaro
release 17.04 is supported.

Additionally, the following fixes are made to the User Guide:
- Removed out of date reference to Linaro release 16.06.
- Updated the Juno variant coverage to include r2.

Change-Id: Iebbced3356f8c6b3c2bff2df62574db9f937ca7b
Signed-off-by: David Cunado <david.cunado@arm.com>