drivers: mmc: Add sdhci driver for Broadcom iProc platform

Add SDHCI driver for iProc family of Broadcom devices.

Signed-off-by: Corneliu Doban <corneliu.doban@broadcom.com>
Signed-off-by: Pramod Kumar <pramod.kumar@broadcom.com>
Signed-off-by: Pavithra Ravi <pavithra.ravi@broadcom.com>
Signed-off-by: Bharat Kumar Reddy Gooty <bharat.gooty@broadcom.com>
Signed-off-by: Vladimir Olovyannikov <vladimir.olovyannikov@broadcom.com>
Signed-off-by: Arun Parameswaran <arun.parameswaran@broadcom.com>