tty/serial: convert 8250 to generic earlycon

With the generic earlycon infrastructure in place, convert the 8250
early console to use it.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>