lib: utils/serial: Support Synopsys DesignWare APB UART

Synopsys DesignWare APB UART is seen on the StarFive JH7100 SoC.
Its programming interface is compatible with the existing 8250
UART driver. Simply add its compatible string to the driver makes
it work with the StarFive JH7100 SoC on a BeagleV board.

With this patch, the generic platform firmware can be used out of
the box on the BeagleV board.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>