nfp: add CPP access core

Command Push Pull is the name of NFP's network on a chip.
PCIe PF can access the interconnect through a number of mappings
controlled via Base Access Registers.  BARs allow the PF to issue
pretty much any command or address any memory on the chip.

Add appropriate logic and a handful of helper for simple operations
like reading scalars from memories.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>