Documentation: msm: imem: snapshot of imem documentation file

This is snapshot of the imem documentation file as of msm-4.4
commit 'fb5706b7462b0b855d10da12ed88ebf10a2e8643'.

Change-Id: I6f3e837e414598e2e8d7119e67b666e33d8af686
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>