mmc: core: Add option to prevent eMMC sleep command

Host may now use MMC_CAP2_NO_SLEEP_CMD to disable the use
of eMMC sleep/awake command.

This option can be used when your platform has a buggy
kernel crash dump software, which is supposed to store
the dump on the eMMC, but is not able to wake up the eMMC
from sleep state.

In particular, failures have been seen with u-boot; even if
it is fixed there, platforms will be slow to update their
bootloader binaries.

Signed-off-by: Ulf Hansson <ulf.hansson@stericsson.com>
Reviewed-by: Hanumath Prasad <hanumath.prasad@stericsson.com>
Reviewed-by: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Acked-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Chris Ball <cjb@laptop.org>