clk: Change bcm clocks build dependency

The clock code under drivers/clk/bcm now contains code for both the
Broadcom mobile SoCs and the iProc SoCs. Change the the makefile
dependency to be under config flag CONFIG_ARCH_BCM that's enabled for
both families of SoCs

Signed-off-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>