msm: ipa: added debugfs support to dump ipa nat table

Currently there is no way to dump ipa nat dynamically using
a command. So added debug fs ip4_nat to dump ipa nat table
dynamically using "cat /sys/kernel/debug/ipa/ip4_nat".
With this change, users/testers can view ipa nat table using
above command.

Change-Id: Ia30b2371c5737e9cee5e42450377372637fff11d
Signed-off-by: Ravi Gummadidala <rgummadi@codeaurora.org>