mmc: enable TRIM/ERASE caps for SDHCI host

Hi Chris,
These patches enable TRIM/ERASE capability for SDHCI host controller.

patch001: mmc: set max_discard_sectors value for mmc queue Rightnow max_discard_sectors value for mmc queue is UINT_MAX which means block layer can accept unlimited sectors to erase at one time. But this may delay some other important I/O requests. So set a suitable value for max_discard_sectors value by using 'pref_erase'

patch0002: mmc: enable TRIM/ERASE capability for SDHCI host. ERASE command needs R1B response, so before sending command, SDHCI host driver also need to set timeout control register for such command

patch0003: mmc: fix division by zero when calculate erase timeout when enable clock gating feature, host clock may be zero when calculate erase timeout value and this will cause division by zero bug.

Thanks
Chuanxiao