mmc: add more trace logs and debug information

Add MMC_TRACE() events in gating and scaling path.
Also, add last successful and failed request timestamps
in struct mmc_host.

Change-Id: I22a138c073624136b858d4fa320aa0ff2d1d2c21
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>