msm: ipc_logging: enhance log-extraction support

One of the main goals of the IPC Logging is to allow extracting logs
from memory dumps.  The current implementation has the following
limitations:

  1) if logs are being dumped through debugfs at the time of the crash,
     it is not possible to extract logs that were already dumped

  2) it is not always possible to tell the difference between empty,
     partially full, and full log pages resulting in extraction
     complications

  3) if the system memory is not cleared between reboots, then the same
     log ID and log page may be extracted leading to duplicate log
     pages

Add additional debugfs read index, timestamps for log pages, and change
the read/write indices to easily differentiate between empty, partially
full, and full log pages.

CRs-Fixed: 670345
Change-Id: I695f6e0605f3755b868ede59851f5ca919f8fb0d
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>