ARM: bcm: provide common arch init for DT clocks

With arch/arm calling of_clk_init(NULL) and clocksource_of_init()
this is no longer needed. The former is useful because we can make
use of dummy fixed clocks for drivers until the bcm281xx common
clock driver is ready.

Signed-off-by: Matt Porter <matt.porter@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Acked-by: Christian Daudt <csd@broadcom.com>