serial: ns16550: Rename serial.c to serial_ns16550.c

This serial driver had wrong name of the source file for some time
now. The name of the driver was serial.c instead of any more logical
and fitting name. Thus, rename the driver source file to serial_ns16550.c
and be done with it.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Michal Simek <monstr@monstr.eu>