mmc: msm_sdcc: clean up sysfs attributes

Currently SDCC driver is using couple of sysfs attributes but
data for these attributes is global so shared by all the
SDCC host controllers but for example, one sysfs attribute named
"polling" is required only for few slots. But as all attribute data
is global, all the slots are getting the "polling" attribute even
if it's not required for few slots.

This change adds attribute data into the host controller (msmsdcc_host)
structure so each host controller can add the required attributes
based on their need.

Change-Id: I06aabf6b74288c2653d4655bf257d1a9af63dc47
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>