mmc: sdhci-msm: Add polling sysfs entry

Add support for polling by providing sysfs entry. It can be
enabled/disabled, by writing 1/0 respectively to the sysfs node -
sys/bus/platform/devices/msm_sdcc.<slotno>/polling. The polling
will be available only if hardware based card detection is not
supported.

Change-Id: Ic58c36665e23cb921d76c482494a168289e83b83
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>