iwlwifi: move PNVM implementation to common code

The PNVM code is generic and can be used by other opmodes.  Move it to
a common file and include it in the relevant opmodes.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20201008181047.232aa310693b.I03a18ffa4162753af38e759d88e27509007c7bca@changeid