clk: bcm2835: Protect sections updating shared registers

CM_PLLx and A2W_XOSC_CTRL registers are accessed by different clock
handlers and must be accessed with ->regs_lock held.
Update the sections where this protection is missing.

Fixes: 41691b88 ("clk: bcm2835: Add support for programming the audio domain clocks")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>