thunderbolt: Split NVM read/write generic functions out from usb4.c

We do this for Thunderbolt 2/3 devices through DMA port, USB4 devices
and retimers pretty much the same way. Only the actual block read/write
is different. For this reason split out the NVM read/write functions
from usb4.c to nvm.c and make USB4 device code call these when needed.

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