efi: Auto-load the efi-pstore module

efi-pstore should be auto-loaded on EFI systems, same as efivars.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Lee, Chun-Yi <jlee@suse.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>