lrdp-v2: mmcdoctor: Add support to extract mmc ring buffer.

Starting from sdm660(kernel version 4.4) mmc driver keeps last 64
mmc trace events in ring buffer.
This patch adds the support to extract it to mmcreport.txt
for both mmc0/mmc1.

Change-Id: I4b1d95106cbdd7262afe604a289ce2ebc59ccc52