mfd: ls2kbmc: Introduce Loongson-2K BMC core driver

The Loongson-2K Board Management Controller provides an PCIe interface
to the host to access the feature implemented in the BMC.

The BMC is assembled on a server similar to the server machine with
Loongson-3 CPU. It supports multiple sub-devices like DRM and IPMI.

Co-developed-by: Chong Qiao <qiaochong@loongson.cn>
Signed-off-by: Chong Qiao <qiaochong@loongson.cn>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Acked-by: Corey Minyard <corey@minyard.net>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Link: https://lore.kernel.org/r/0dc1fd53020ce2562453961ffed2cd9eb8f359e1.1756987761.git.zhoubinbin@loongson.cn


Signed-off-by: Lee Jones <lee@kernel.org>