The RPi firmware-based clocks driver can actually disable
unused clocks, so when switching to use it we ended up losing
our MMC clock once all devices were probed.

This patch adopts the changes from 1e5a0a9a ("mmc: sdhci-bcm2835:
Actually enable the clock") to sdhci-iproc.
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>