mmc: DT: sdhci-iproc: add bcm2835 compatible

Since sdhci-iproc can support bcm2835 we need add the
compatible string to the binding.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Scott Branden <sbranden@broadcom.com>
Acked-by: Eric Anholt <eric@anholt.net>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Scott Branden <sbranden@broadcom.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>