Rationalize UART usage among different BL stages

This patch changes the UART port assignment for various BL stages
so as to make it consistent on the platform ports. The BL1, BL2 and
BL3-1 now uses UART0 on the FVP port and SoC UART0 on the Juno port.
The BL3-2 uses UART2 on the FVP port and FPGA UART0 on the Juno
port.

This provides an interim fix to ARM-software/tf-issues#220 until
support is added for changing the UART port for a BL image between
cold boot and runtime.

Change-Id: Iae5faea90be3d59e41e597b34a902f93e737505a