memshare: Add snapshot of memshare dt documentation

The patch is a snapshot of memshare driver dt documentation
taken from msm-4.14 commit <460b918c8d6d9>
(Merge "net: aquantia: support for 32-bit ARM target").

Change-Id: I52c85cedd981b085c4c4d970c7631727a501c9de
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>