serial: 8250_uniphier: add earlycon support

This reuses the code of drivers/tty/serial/8250/8250_early.c except

  - Overwrite device->port.iotype and device->port.regshift for
    UPIO_MEM32 because of_setup_earlycon() has set them for UPIO_MEM.

  - Set device->baud to zero to prevent early8250_setup() from
    initializing the divisor register because port->uartclk does not
    match the frequency expected by this hardware.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>