serial: 8250_early: Add earlycon support for Synopsys DesignWare ABP UART

Some board like Hisilicon D02 uses Synopsys DesignWare ABP UART, declare an
OF early console for it, so early console device can be enabled with comand
line "earlycon"(without option) via the "stdout-path" property in device-tree.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Tested-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>