tty/serial: add back missing setup_early_serial8250_console

Commit d2fd6810 (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>

drivers/tty/serial/8250/8250_early.c

@@ -156,6 +156,16 @@ static int __init early_serial8250_setup(struct earlycon_device *device,
 EARLYCON_DECLARE(uart8250, early_serial8250_setup);
 EARLYCON_DECLARE(uart, early_serial8250_setup);
 
+int __init setup_early_serial8250_console(char *cmdline)
+{
+	char match[] = "uart8250";
+
+	if (cmdline && cmdline[4] == ',')
+		match[4] = '\0';
+
+	return setup_earlycon(cmdline, match, early_serial8250_setup);
+}
+
 int serial8250_find_port_for_earlycon(void)
 {
 	struct earlycon_device *device = early_device;