thunderbolt: Split common NVM functionality into a separate file

We are going to reuse some of this functionality to implement retimer
NVM upgrade so move common NVM functionality into its own file. We also
rename the structure from tb_switch_nvm to tb_nvm to make it clear that
it is not just for switches.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>