mmc: queue: Introduce queue depth and use it to allocate and free

Add a mmc_queue member to record the size of the queue, which currently
supports 2 requests on-the-go at a time. Instead of allocating resources
for 2 slots in the queue, allow for an arbitrary number.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>