tty: serial: 8250: export early_serial8250_setup function

8250-like uart driver may call early_serial8250_setup to
reuse 8250_early.c character output function.

Signed-off-by: Eddie Huang <eddie.huang@mediatek.com>
Tested-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>