ARM: common: fiq_debugger: add uart_enable/disable platform callbacks

This allows the platform specific drivers to properly enable
and disable the uart at the appropriate times. On some platforms, just
managing the clock is not enough.

Change-Id: I5feaab04cfe313a4a9470ca274838676b9684201
Signed-off-by: Dima Zavin <dima@android.com>