serial: 8250: Add Loongson uart driver support

serial: 8250: Add Loongson uart driver support

Add the driver for on-chip UART used on Loongson family chips.

The hardware is similar to NS16550A, but there are the following
differences:
 - Some chips (such as Loongson-2K2000) have added a fractional division
   register to obtain the required baud rate accurately, so the
   {get,set}_divisor callback is overridden.
 - Due to hardware defects, quirk handling is required for
   UART_MCR/UART_MSR.

Co-developed-by: Haowei Zheng <zhenghaowei@loongson.cn>
Signed-off-by: Haowei Zheng <zhenghaowei@loongson.cn>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://patch.msgid.link/2c2a01a276b9250efea0c7aa190efecdfd6fdf5a.1760166651.git.zhoubinbin@loongson.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>