clk: Add safe switch hook

Sometimes clocks can't accept their parent source turning off
while the source is reprogrammed to a different rate. Most
notably some CPU clocks require a way to switch away from the
current PLL they're running on, reprogram that PLL to a new rate,
and then switch back to the PLL with the new rate once they're
done.  Add a hook that drivers can implement allowing them to
return a 'safe parent' that they can switch their parent to while
the upstream source is reprogrammed.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>