mmc: sdhci-msm: Make HS400 tuning follow as per recommeneded HW sequence

During tuning execution for HS400 mode, HW sequence recommends
to select MCLK_SEL/2(0x3) in VENDOR_SPEC & sdhc msm clock at GCC
to be 400MHZ (nearest supported clk). Add this change in tuning
sequence during HS400 tuning.

Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>
Tested-by: Jeremy McNicoll <jeremymc@redhat.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>