mmc: core: Add support for Power Off Notification for SD cards

Rather than only deselecting the SD card via a CMD7, before we cut power to
it at system suspend, at runtime suspend or at shutdown, let's add support
for a graceful power off sequence via enabling the SD Power Off
Notification feature.

Note that, the Power Off Notification feature was added in the SD spec
v4.x, which is several years ago. However, it's still a bit unclear how
often the SD card vendors decides to implement support for it. To validate
these changes a Sandisk Extreme PRO A2 64GB has been used, which seems to
work nicely.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Avri Altman <avri.altman@wdc.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Link: https://lore.kernel.org/r/20210504161222.101536-12-ulf.hansson@linaro.org