mmc: host: Add one missing file to support ring buffer logging

Commit 5bf42096 ("mmc: host: Add Ring buffer
logging for MMC.") missed ring buffer C file.
This change adds ring buffer C file to enable
tracing of significant events of eMMC in both
CQ and Legacy mode for debug purpose.

Change-Id: I358cef651863687cc25de965f71fd2821c0a7c79
Signed-off-by: Can Guo <cang@codeaurora.org>