ARM: dts: bcm281xx: define real clocks

Replace the "fake" clocks defined in the "bcm11351.dtsi" device tree
file with real definitions backed by the new BCM281xx clock driver.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Matt Porter <mporter@linaro.org>
Reviewed-by: Tim Kryger <tim.kryger@linaro.org>
Signed-off-by: Matt Porter <mporter@linaro.org>