ARM: dts: use real clocks for bcm21664

Replace the "fake" fixed-rate clocks used previously for the
bcm21664 family with "real" ones.

Signed-off-by: Alex Elder <elder@linaro.org>
Acked-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>