Platform: Unify NS and secure UART STDOUT implementation

The uart_stdout implementations are actually identical for
Non-secure and Secure. The only difference is the actual
underlying stdio driver.
This patch unifies the implementations to one file, using
Macro to distinguish different drivers.

The patch also removes the unused enum uart_channel.

Change-Id: Ice89127cc98bd185947ce8de7af5190bd5bd69dc
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
5 files changed