clk: Kconfig: Move bcm Kconfig into clk menu

Having this Kconfig sourced outside the clk menu means the option
is under the "Device Drivers" menu instead of the "Common Clock
Framework" menu. Move it so that the bcm clock config options are
in the right place.

Cc: Alex Elder <elder@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>