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>
ARM: dts: bcm281xx: change comment to C89 style

CodingStyle does not allow C99 style comments. Since the
dts files live in the kernel for now, make this compliant.

Signed-off-by: Matt Porter <matt.porter@linaro.org>
Acked-by: Christian Daudt <csd@broadcom.com>