edac: move documentation from edac_mc.c to edac_core.h

Several functions are documented at edac_mc.c.

As we'll be including edac_core.h at drivers-api book, move
those, in order for the kernel-doc markups be part of the API
documentation book.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>