tty/serial: add back missing setup_early_serial8250_console

tty/serial: add back missing setup_early_serial8250_console

Commit d2fd6810a823bcd (tty/serial: convert 8250 to generic earlycon)
removed setup_early_serial8250_console, but there are still 2 callers
in:

arch/mips/mti-malta/malta-init.c
drivers/firmware/pcdp.c

Add back the function implemented as a wrapper to setup_earlycon.

Reported-by: Yinghai Lu <yinghai@kernel.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: linux-serial@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit fe1cf8af918af3ff0dd58ce92e5a5da117cb1d92)
Signed-off-by: Alex Shi <alex.shi@linaro.org>
(cherry picked from commit bd3132993d81a28b3a165ef6cc1abe0c4a0c6ac6)
Signed-off-by: Alex Shi <alex.shi@linaro.org>