tracing/power: move the power trace headers to a dedicated file

Impact: cleanup

Move the power tracer headers to trace/power.h to keep ftrace.h and power bits
more easy to maintain as separated topics.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>