mmc: dw_mmc: Add support for pre_req and post_req

This patch implements pre_req and post_req in dw_mmc to support
asynchronous mmc request.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>