mmc: sdhci-msm: Add dbg_state sysfs

Add dbg_state sysfs to facilitate trace instance to be enabled
only on debug flavor builds.

Change-Id: I6524e6184e2a81b2e39b0f0338e4d8a9b7989542
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>