mmc: support BKOPS feature for eMMC

Enable eMMC background operations (BKOPS) feature.

If URGENT_BKOPS is set after a response, note that BKOPS are required.
Immediately run BKOPS if required.  Read/write operations should be
requested during BKOPS(LEVEL-1), then issue HPI to interrupt the
ongoing BKOPS and service the foreground operation.
(This patch only controls the LEVEL2/3.)

When repeating the writing 1GB data, at a certain time, performance is
decreased.  At that time, card triggers the Level-3 or Level-2.  After
running bkops, performance is recovered.

Future considerations:
 * Check BKOPS_LEVEL=1 and start BKOPS in a preventive manner.
 * Interrupt ongoing BKOPS before powering off the card.
 * How do we get BKOPS_STATUS value (periodically send ext_csd command)?
 * If using periodic bkops, also consider runtime_pm control.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>
Reviewed-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Chris Ball <cjb@laptop.org>