net: ipa: introduce sysfs code

Add IPA device attributes to expose information known by the IPA
driver about the hardware and its configuration.

All pointers used to display these attribute values (i.e., IPA
pointer and endpoint pointers) will have been initialized by the
time IPA probe has completed, so they may be safely dereferenced.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>