nfp: support long reads and writes with the cpp helpers

nfp_cpp_{read,write}() helpers perform device memory mapping (setting
the PCIe -> NOC translation BARs) and accessing it.  They, however,
currently implicitly expect that the length of entire operation will
fit in one BAR translation window.  There is a number of 16MB windows
available, and we don't really need to access such large areas today.

If the user, however, manages to trick the driver into making a big
mapping (e.g. by providing a huge fake FW file), the driver will
print a warning saying "No suitable BAR found for request" and a
stack trace - which most users find concerning.

To be future-proof and not scare users with warnings, make the
nfp_cpp_{read,write}() helpers do accesses chunk by chunk if the area
size is large.  Set the notion of "large" to 2MB, which is the size
of the smallest BAR window.

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