ARM: bL_switcher: Basic trace events support

This patch adds simple trace events to the b.L switcher code
to allow tracing of CPU migration events.

To make use of the trace events, you will need:

CONFIG_FTRACE=y
CONFIG_ENABLE_DEFAULT_TRACERS=y

The following events are added:
  * power:cpu_migrate_begin
  * power:cpu_migrate_finish

each with the following data:
    u64     timestamp;
    u32     cpu_hwid;

power:cpu_migrate_begin occurs immediately before the
switcher-specific migration operations start.
power:cpu_migrate_finish occurs immediately when migration is
completed.

The cpu_hwid field contains the ID fields of the MPIDR.

* For power:cpu_migrate_begin, cpu_hwid is the ID of the outbound
  physical CPU (equivalent to (from_phys_cpu,from_phys_cluster)).

* For power:cpu_migrate_finish, cpu_hwid is the ID of the inbound
  physical CPU (equivalent to (to_phys_cpu,to_phys_cluster)).

By design, the cpu_hwid field is masked in the same way as the
device tree cpu node reg property, allowing direct correlation to
the DT description of the hardware.

The timestamp is added in order to minimise timing noise.  An
accurate system-wide clock should be used for generating this
(hopefully getnstimeofday is appropriate, but it could be changed).
It could be any monotonic shared clock, since the aim is to allow
accurate deltas to be computed.  We don't necessarily care about
accurate synchronisation with wall clock time.

In practice, each switch takes place on a single logical CPU,
and the trace infrastructure should guarantee that events are
well-ordered with respect to a single logical CPU.

Signed-off-by: Dave Martin <dave.martin@linaro.org>
Signed-off-by: Nicolas Pitre <nico@linaro.org>