tty: serial: lantiq: implement earlycon support

This allows enabling earlycon for devices with a Lantiq serial console
by splitting lqasc_serial_port_write() from lqasc_console_write() and
re-using the new function for earlycon's write callback.

The kernel-parameter name matches the driver name ("lantiq"), similar
to how other drivers implement this.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>