serial: Move common/serial.c to drivers/serial/

Move the common/serial.c into driver/serial/, since this file
provides serial multiplexing functions and it is imperative to
be linked with libserial.o instead of libcommon.o.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>