mmc: sdhci: Add quirk to disable LED control

Host controllers may not have a LED signal line, whereas the register
updates to control the non-existent LED can be relatively time consuming.
Add a quirk to disable LED control.

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