Fix serial driver

Why i commit this patch?
    first, i use mutiple serial device, but serial buffer size is the same, i can not modify it for each one.
    second, i can not use hardware flow function for serial device.
    third, the uart device driver is bad support for stm32 gpio remap, and can not support uart4, uart5.
    fourth, the serial device driver use old ring buffer implementation.
so, i fix it, and it is work fine.