Add SDHCI bindings for the Broadcom 281xx SoCs.

Changes from V2:
 - Documentation cleanups

Changes from V1:
 - split original patch into 2, one for driver and this one for dt
Signed-off-by: NChristian Daudt <csd@broadcom.com>
Acked-by: NArnd Bergmann <arnd@arndb.de>