dt-bindings: serial: 8250: Add Loongson uart compatible

dt-bindings: serial: 8250: Add Loongson uart compatible

The Loongson family have a mostly NS16550A-compatible UART and
High-Speed UART hardware with the exception of custom frequency divider
latch settings register.

Co-developed-by: Haowei Zheng <zhenghaowei@loongson.cn>
Signed-off-by: Haowei Zheng <zhenghaowei@loongson.cn>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://patch.msgid.link/2d858e9303d95a3e4909aa9c1379d4abbdc52cc2.1760166651.git.zhoubinbin@loongson.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>