efi: Register efi_secret platform device if EFI secret area is declared

During efi initialization, check if coco_secret is defined in the EFI
configuration table; in such case, register platform device
"efi_secret". This allows udev to automatically load the efi_secret
module (platform driver), which in turn will populate the
<securityfs>/secrets/coco directory in guests into which secrets were
injected.

Note that a declared address of an EFI secret area doesn't mean that
secrets where indeed injected to that area; if the secret area is not
populated, the driver will not load (but the platform device will still
be registered).

Signed-off-by: Dov Murik <[e-mail]>
Reviewed-by: Gerd Hoffmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Ard Biesheuvel <[e-mail]>

drivers/firmware/efi/efi.c

···
 	if (efi_enabled(EFI_DBG) && efi_enabled(EFI_PRESERVE_BS_REGIONS))
 		efi_debugfs_init();
 
+#ifdef CONFIG_EFI_COCO_SECRET
+	if (efi.coco_secret != EFI_INVALID_TABLE_ADDR)
+		platform_device_register_simple("efi_secret", 0, NULL, 0);
+#endif
+
 	return 0;
 
 err_remove_group: