mmc: set a suitable max discard sectors value for host controller

Hi Chris,
These 3 patches set a suitable max_discard_sectors value for mmc queue which will be used for ERASE/TRIM. Add Adrian in Cc loop since he implemented the ERASE/TRIM feature for MMC driver.

patch1: calculated a suitable timeout time for SDHCI host controller. Each erase cmd should be finished within the timeout time.

patch2: calculated a suitable max_discard_sectors value for mmc queue. Since some SDHCI host controller cannot process the whole sectors at one time. In this patch, a new callback get_tmclk in mmc_host_ops was added to help core layer to calculate the max value. If this callback was implemented, core layer will calculted a suitable value. If not, the max_discard_sectors value will be the same as before.

patch3: implemented get_tmclk callback for SDHCI host controller.

Adrian, hope I can get your comment about these patches as well. Thanks for your time to review these patches.

Regards
Chuanxiao