dt-bindings: Add binding for brcm,bcm2835-sdhost.

This is the other SD controller on the platform, which can be swapped
to the role of SD card host using pin muxing.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>