clk: bcm2835: Mark GPIO clocks enabled at boot as critical.

These divide off of PLLD_PER and are used for the ethernet and wifi
PHYs source PLLs.  Neither of them is currently represented by a phy
device that would grab the clock for us.

This keeps other drivers from killing the networking PHYs when they
disable their own clocks and trigger PLLD_PER's refcount going to 0.

v2: Skip marking as critical if they aren't on at boot.

Signed-off-by: Eric Anholt <eric@anholt.net>