ARM: mmc: bcm281xx SDHCI driver (dt mods)

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: Christian Daudt <csd@broadcom.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>