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

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>
(cherry picked from commit 3f9195811d8d829556c4cd88d3f9e56a80d5ba60)