mmc: host: Add Ring buffer logging for MMC.

Add ring buffer to enable tracing of significant events
of eMMC in both CQ and Legacy mode for debug purpose.

CONFIG_MMC_RING_BUFFER should be enabled to use ring
buffer logging.

Change-Id: Iceb404d77fca19e133c319bc66ec15b2d77c7c54
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>
[xiaonian@codeaurora.org: fixed trivial merge conflicts]
Signed-off-by: Xiaonian Wang <xiaonian@codeaurora.org>