It is not ideal for BL31 to continue to use boot console at
runtime which could be potentially uninitialized. This patch
introduces a new optional platform porting API
`bl31_plat_runtime_setup()` which allows the platform to perform
any BL31 runtime setup just prior to BL31 exit during cold boot.
The default weak implementation of this function will invoke
`console_uninit()` which will suppress any BL31 runtime logs.

On the ARM Standard platforms, there is an anomaly that
the boot console will be reinitialized on resumption from
system suspend in `arm_system_pwr_domain_resume()`. This
will be resolved in the following patch.

NOTE: The default weak definition of `bl31_plat_runtime_setup()`
disables the BL31 console. To print the BL31 runtime
messages, platforms must override this API and initialize a
runtime console.

Fixes ARM-software/tf-issues#328

Change-Id: Ibaf8346fcceb447fe1a5674094c9f8eb4c09ac4a