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 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' and 'safe frequency' that they can switch their
parent to while the upstream source is reprogrammed to support
this.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>