pl011: allow alternate stdout to be used

Till now only pl011 UART is available stdout but there is a usecase to
redirect it to other channel.

This patch decouples console_putc implementation from pl011.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I38e8c5c90421fad2425228f407e6f29bdf6b08c7