platform: andes/ae350: Use fdt serial driver

Andes UART is compatible with uart8250 driver. We can use
fdt_serial_init() as platform console init hook.

dts example:

  serial0: serial@f0300000 {
    compatible = "andestech,uart16550", "ns16550a";
    reg = <0x00000000 0xf0300000 0x00000000 0x00001000>;
    interrupts = <9 4>;
    interrupt-parent = <&plic0>;
    clock-frequency = <19660800>;
    current-speed = <38400>;
    reg-shift = <2>;
    reg-offset = <32>;
    reg-io-width = <4>;
    no-loopback-test = <1>;
  };

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>