fiq_debugger: Add option to apply uart overlay by FIQ_DEBUGGER_UART_OVERLAY

fiq_debugger is taking over uart, so it is necessary to disable
original uart in DT file. It can be done manually or by overlay.

Change-Id: I9f50ec15b0e22e602d73b9f745fc8666f8925d09
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>