clk: Add support for enabling/disabling clocks from debugfs

For test and debug purposes, it's simple enough to enable or disable
clocks from shell. Add a new debugfs file 'clk_prepare_enable' that
calls clk_prepare_enable() when writing "1" and clk_disable_unprepare()
when writing "0".

This can have security implications, so only support it when the code
has been modified to #define CLOCK_ALLOW_WRITE_DEBUGFS.

Signed-off-by: Mike Tipton <mdtipton@codeaurora.org>
Link: https://lore.kernel.org/r/20200630003024.6282-1-mdtipton@codeaurora.org


[sboyd@kernel.org: Reword commit text and remove comment update]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>