dt-bindings: clk: Introduce 'protected-clocks' property

Add a generic clk property for clks which are not intended to be used by
the OS due to security restrictions put in place by firmware. For
example, on some Qualcomm firmwares reading or writing certain clk
registers causes the entire system to reboot, but on other firmwares
reading and writing those same registers is required to make devices
like QSPI work. Rather than adding one-off properties each time a new
set of clks appears to be protected, let's add a generic clk property to
describe any set of clks that shouldn't be touched by the OS. This way
we never need to register the clks or use them in certain firmware
configurations.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Taniya Das <tdas@codeaurora.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>