mmc: core: add non-blocking mmc request function

mmc: core: add non-blocking mmc request function

Previously there has only been one function mmc_wait_for_req()
to start and wait for a request. This patch adds:

 * mmc_start_req() - starts a request wihtout waiting
   If there is on ongoing request wait for completion
   of that request and start the new one and return.
   Does not wait for the new command to complete.

This patch also adds new function members in struct mmc_host_ops
only called from core.c:

 * pre_req - asks the host driver to prepare for the next job
 * post_req - asks the host driver to clean up after a completed job

The intention is to use pre_req() and post_req() to do cache maintenance
while a request is active. pre_req() can be called while a request is
active to minimize latency to start next job. post_req() can be used after
the next job is started to clean up the request. This will minimize the
host driver request end latency. post_req() is typically used before
ending the block request and handing over the buffer to the block layer.

Add a host-private member in mmc_data to be used by pre_req to mark the
data. The host driver will then check this mark to see if the data is
prepared or not.

Signed-off-by: Per Forlin <per.forlin@linaro.org>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Venkatraman S <svenkatr@ti.com>
Tested-by: Sourav Poddar <sourav.poddar@ti.com>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>