mfd: intel-m10-bmc: Split into core and spi specific parts

Split the common code from intel-m10-bmc driver into intel-m10-bmc-core
and move the SPI bus parts into an interface specific file.

intel-m10-bmc-core becomes the core MFD functions which can support
multiple bus interface like SPI bus.

Co-developed-by: Tianfei zhang <tianfei.zhang@intel.com>
Signed-off-by: Tianfei zhang <tianfei.zhang@intel.com>
Reviewed-by: Russ Weight <russell.h.weight@intel.com>
Acked-by: Guenter Roeck <linux@roeck-us.net> # hwmon
Reviewed-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230116100845.6153-5-ilpo.jarvinen@linux.intel.com