EDAC/skx_common: Move mc_mapping to be a field inside struct skx_imc

EDAC/skx_common: Move mc_mapping to be a field inside struct skx_imc

The mc_mapping and imc fields of struct skx_dev have the same size,
NUM_IMC. Move mc_mapping to be a field inside struct skx_imc to prepare
for making the imc array of memory controller instances a flexible array.

No functional changes intended.

Suggested-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20250731145534.2759334-3-qiuxu.zhuo@intel.com