Update dependencies for ARM TF

- Linaro binaries:    18.04
- mbed TLS library:   2.12.0
- FVP model versions: 11.4 build 37

This patch updates the user guide documentation to reflect these
changes to the dependencies.

Change-Id: I454782ca43a0db43aeeef2ab3622f4dea9dfec55
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>