efi: expose efivar generic ops register function

This is a preparation for supporting efivar operations provided by other
than efi subsystem.  Both register and unregister functions are exposed
so that non-efi subsystem can revert the efi generic operation.

Acked-by: Sumit Garg <sumit.garg@linaro.org>
Co-developed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>