thunderbolt: Provide tb_retimer_nvm_read() analogous to tb_switch_nvm_read()

As we are moving the NVM vendor specifics into nvm.c we need to deal
witht he retimer NVM formats too. For this reason provide retimer
specific function that can be used to read the contents of the NVM and
rename the internal ones accordingly analogous to what we do with
routers.

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