serial: Implement serial_initfunc() macro

This macro simplifies declaration of weak aliases for per-driver
functions, which register these drivers with the serial subsystem.
The idea here is to push serial_register() calls from serial.c into
the drivers. To avoid pile of ifdef construct as it is now, create
weak aliases to these functions, which in case the driver is not
present alias onto an empty function, which is in turn optimized out
altogether.

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>