mmc: dw_mmc: Add missing description

Commit 0976f16d ("mmc: dw_mmc: add support tuning scheme") introduced
the execute_tuning hook but did not add its description for kernel docs.
Update the same.

Signed-off-by: Sachin Kamat <[e-mail]>
Acked-by: Jaehoon Chung <[e-mail]>
Acked-by: Seungwon Jeon <[e-mail]>
Signed-off-by: Chris Ball <[e-mail]>

drivers/mmc/host/dw_mmc.h

···
  * @prepare_command: handle CMD register extensions.
  * @set_ios: handle bus specific extensions.
  * @parse_dt: parse implementation specific device tree properties.
+ * @execute_tuning: implementation specific tuning procedure.
  *
  * Provide controller implementation specific extensions. The usage of this
  * data structure is fully optional and usage of each member in this structure