Add missing locking to:
* bcm2835_pll_divider_on
* bcm2835_pll_divider_off
to protect the read modify write cycle for the
register access protecting both cm_reg and a2w_reg
registers.
Fixes: 41691b8862e2 ("clk: bcm2835: Add support for programming the
audio domain clocks")
Signed-off-by: Martin Sperl <kernel@martin.sperl.org> Signed-off-by: Eric Anholt <eric@anholt.net> Reviewed-by: Eric Anholt <eric@anholt.net>