efi: rtc-efi: Export platform:rtc-efi as module alias

When the rtc-efi driver is built as a module, we already register the
EFI rtc as a platform device if UEFI Runtime Services are enabled.
To wire it up to udev, and let the module be loaded automatically, we
need to export the 'platform:rtc-efi' alias from the module.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Mark Salter <msalter@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>