ARM: dts: msm: Increase memory for diag client on sdm660

Currently, memshare allocates 5MB memory to diag client.
This patch increases the memory to 15MB. Also now diag
client will be considered guaranteed to avoid memory
allocation failure on modem SSRs.

Change-Id: I2ac822ef95ab7fbf054fdc80052b8406732f383e
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>