mmc: sdhci-msm: Add platform_execute_tuning implementation

This patch adds implementation for platform specific tuning in order
to support HS200 bus speed mode on Qualcomm SDHCI controller.

Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>
Signed-off-by: Georgi Djakov <gdjakov@mm-sol.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>