Rename the serial_register_bfin_uart() to bfin_initialize_serial()
to be consistent with the rest of the naming. Next, remove it's
prototype from serial.h and properly insert it into serial.c as
the rest of the serial initialization functions.

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>