serial: push default_serial_console to drivers

Rather than sticking arch/board/driver specific logic in the common
serial code, push it all out to the respective drivers.  The serial
drivers declare these funcs weak so that boards can still override
things with their own definition.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
CC: Heiko Schocher <hs@denx.de>
CC: Anatolij Gustschin <agust@denx.de>
CC: Tom Rix <Tom.Rix@windriver.com>
CC: Minkyu Kang <mk7.kang@samsung.com>
CC: Craig Nauman <cnauman@diagraph.com>
CC: Prafulla Wadaskar <prafulla@marvell.com>
CC: Mahavir Jain <mjain@marvell.com>
Tested-by: Minkyu Kang <mk7.kang@samsung.com>