spi: loongson: add bus driver for the loongson spi controller

This bus driver supports the Loongson SPI hardware controller in the
Loongson platforms and supports the use DTS and PCI framework to
register SPI device resources.

Signed-off-by: Yinbo Zhu <zhuyinbo@loongson.cn>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20230613075834.5219-3-zhuyinbo@loongson.cn
Signed-off-by: Mark Brown <broonie@kernel.org>