serial: bfin_sport_uart: add support for CTS/RTS via GPIOs

Some people need flow control on their ports, so now boards can support
that via any GPIOs.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>