mmc: sd: Export SD Status via “ssr” device attribute

The SD Status register contains several important fields related to the
SD Card proprietary features.
Those fields may be used by user space applications for vendor specific
usage.
None of those fields are exported today by the driver to user space.
In this patch, we are reading the SD Status register and exporting
(using MMC_DEV_ATTR) the SD Status register to the user space.

Signed-off-by: Uri Yanai <uri.yanai@sandisk.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>