pwm: Add Loongson PWM controller support

This commit adds a generic PWM framework driver for the PWM controller
found on Loongson family chips.

Acked-by: Huacai Chen <chenhuacai@loongson.cn>
Co-developed-by: Juxin Gao <gaojuxin@loongson.cn>
Signed-off-by: Juxin Gao <gaojuxin@loongson.cn>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Link: https://lore.kernel.org/r/76050a903a8015422fb9261ad88c7d9cc2edbbd8.1743403075.git.zhoubinbin@loongson.cn


Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>