efi/arm: Start using fw_devlink_get_flags()

The fw_devlink_get_flags() provides the right flags to use when creating
mandatory device links derived from information provided by the
firmware. So, use that.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/20200222014038.180923-4-saravanak@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>