mmc: msm_sdcc: enable support to handle Urgent data transfer request

In order to decrease a latency of a prioritized requests (aka Urgent
request), we might want to stop the transmission of a running "low
priority" request in order to handle the Urgent request.

URGENT_REQUEST handling has following dependencies:
1. Host controller driver should support mmc_host op named "stop_request"
2. Block I/O scheduler should support re-insert API
3. eMMC card should support HPI (High Priority Interrupt) command
If any of the above dependencies are not met then urgent request mechanism
will not become operational.

Change-Id: Ic3e5f587b3d00a9cb717290ce1a7926c8c3acd70
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>