clk: bcm2835: enable management of PCM clock

Enable the PCM clock in the SOC, which is used by the
bcm2835-i2s driver.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>