mmc: block: add discard and secdiscard support for CMDQ mode

Discard is supported in CMDQ mode only when device queue is empty.
Hence, discard commands should be sent using DCMD slot with
QBR (Queue Barrier) flag set.

Change-Id: I630091cbd94ffcdcec71626257f912c15fd2e21e
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>