trace: power: add trace_clock_set_parent

Adds a new trace event to be called from clk_set_parent.  Some
cpufreq drivers, including Tegra, reparent the cpu clock to a
slower clock while the main pll is relocking, tracing
clk_set_parent allows traces to show how for long the cpu is
running slower.

Uses a separate TRACE_EVENT instead of the clock event class to
allow the event to contain string names for the child and the
parent.

Signed-off-by: Colin Cross <ccross@android.com>