MdeModulePkg/SdMmcPciHcDxe: Add function to start SD clock

MdeModulePkg/SdMmcPciHcDxe: Add function to start SD clock

In SD card voltage switch flow we used to redo the
entire internal clock setup after voltage switch.
Since internal clock has already been setup this
is wasting time on polling the internal clock stable.
This commit changes it to only start the SD clock.

Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Marcin Wojtas <mw@semihalf.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Mateusz Albecki <mateusz.albecki@intel.com>
Tested-by: Marcin Wojtas <mw@semihalf.com>
Tested-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>