As per the Host Controller Standard Specification Version 4.20,
limitation of tuning iteration count is removed as PLL locking
time can be longer than UHS-1 tuning due to larger PVT fluctuation
and it will result in increase of tuning iteration to complete the
tuning.

This patch creates sdhci_host member tuning_loop_count to allow
hosts to specify maximum tuning iterations and also updates
execute_tuning to use this specified maximum tuning iteration count.

Default tuning_loop_count is set to same as existing loop count of
MAX_TUNING_LOOP which is 40 iterations.
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>