rpi3: add RPI3_RUNTIME_UART build option

Some OSes (e.g. Ubuntu 18.04 LTS on Raspberry Pi 3) may disable the
runtime UART in a manner that prevents the system from rebooting if
ATF tries to send runtime messages there.

Also, we don't want the firmware to share the UART with normal
world, as this can be a DoS attack vector into the secure world.

This patch fixes these 2 issues by introducing new build option
RPI3_RUNTIME_UART, that disables the runtime UART by default.

Fixes ARM-software/tf-issues#647

Signed-off-by: Pete Batard <pete@akeo.ie>