thunderbolt: Add support for separating the flush to SPI and authenticate

This allows userspace to have a shorter period of time that the device
is unusable and to call it at a more convenient time.

For example flushing the image may happen while the user is using the
machine and authenticating/rebooting may happen while logging out.

Signed-off-by: Mario Limonciello <mario.limonciello@dell.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>