clk: bcm2835: Add leaf clock measurement support, disabled by default

This proved incredibly useful during debugging of the DSI driver, to
see if our clocks were running at rate we requested.  Let's leave it
here for the next person interacting with clocks on the platform (and
so that hopefully we can just hook it up to debugfs some day).

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>