i2c: ls2x: Add driver for Loongson-2K/LS7A I2C controller

This I2C module is integrated into the Loongson-2K SoCs and Loongson
LS7A bridge chip.

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>