Remove the exports from serial.h as they are only used in the blackfin
serial driver. Furthermore, they are only used for registration, which
is handled already inside that driver and default_serial_port() call,
which is also handled in that driver.

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