clk: bcm281xx: don't disable unused peripheral clocks

Add the CLK_IGNORE_UNUSED flag when setting up a peripheral clock.
This prevents unused clocks from getting disabled, and by doing
this we can use the common clock code even before we've resolved
all the spots that need to get a reference to their clock.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Matt Porter <mporter@linaro.org>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Matt Porter <mporter@linaro.org>