EDAC/ghes: Prepare to make ghes_edac a proper module

To make ghes_edac a proper module, prepare to decouple its dependencies
from GHES.

Move the ghes_edac.force_load parameter to ghes.c in order to
properly control whether ghes_edac should be force-loaded: In
ghes_edac_register() it is too late to set the module flag.

Introduce a helper ghes_get_devices(), which returns the list of GHES
devices which got probed when the platform-check passes on the system.

The previous force_load check is not needed in ghes_edac_unregister()
since it will be checked in the module's init function of ghes_edac
later.

  [ bp: Massage. ]

Suggested-by: Toshi Kani <toshi.kani@hpe.com>
Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Jia He <justin.he@arm.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221010023559.69655-4-justin.he@arm.com