clk: bcm2835: Mark PLLD_PER as CRITICAL

The VPU firmware assume that the PLLD_PER isn't modified by the ARM core.
Otherwise this could cause firmware lookups. So mark the clock as critical
to avoid this.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Eric Anholt <eric@anholt.net>