mmc: Add bcm2835 sdhost controller

The BCM2835 family of SoCs has 2 different SD controllers: One based on
the SDHCI spec and a custom, home-grown one.

This patch implements a driver for the latter based on the Linux driver.
This is needed so that we can make use of device trees that assume driver
presence of both SD controllers.

Signed-off-by: Alexander Graf <agraf@suse.de>