usb: rmnet: Add timestamps for SKB life cycle in UL and DL directions

This change adds support to keep track of total time spent by a SKB in
UL and DL directions. Timestamps are recorded per SKB for the complete
life cycle at following stages:-

1) Creation: At the time of SKB allocation.
2) Submission: While SKB being submitted to HSIC HW by bridge driver/
   HSUSB Core by rmnet gadget driver.
3) Completion: In completion handler of the driver(bridge driver/
   gadget rmnet driver).
4) Sent: While sending SKB to bridge driver/rmnet gadget driver.
5) Queued: While SKB being queued to HSIC HW by bridge driver/
   HSUSB Core by rmnet gadget driver.
6) Done: In completion handler while freeing the SKB.

debugfs node /sys/kernel/debug/data_hsic_bridge/timestamp represents the
listing of timestamp for last 34 SKBs submitted in UL direction.

debugfs node /sys/kernel/debug/ghsic_data_xport/timestamp represents the
listing of timestamp for last 34 SKBs submitted in DL direction.

Recording of the time stamp is controlled by the module parameter in
rmnet bridge driver and rmnet gadget driver.

To enable the recording of timestamp in rmnet bridge driver
echo 1 > /sys/module/mdm_bridge/parameters/record_timestamp
To enable the recording of timestamp in rmnet gadget driver
echo 1 > /sys/module/g_android/parameters/record_timestamp

Change-Id: I10e7c171eec6a01afbd48328ba006f776370ac92
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>