mmc: sdhci-bcm2835: Actually enable the clock

We're currently using a fixed frequency clock specified in the DT, so
enabling is a no-op.  However, 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.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>