clk: bcm2835: Add binding docs for the new platform clock driver.

Previously we've only supported a few fixed clocks based on
assumptions about how the firmware sets up the clocks, but this
binding will let us control the actual (audio power domain) clock
manager.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Lee Jones <lee@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>