thermal/debugfs: Add thermal cooling device debugfs information

The thermal framework does not have any debug information except a
sysfs stat which is a bit controversial. This one allocates big chunks
of memory for every cooling devices with a high number of states and
could represent on some systems in production several megabytes of
memory for just a portion of it. As the sysfs is limited to a page
size, the output is not exploitable with large data array and gets
truncated.

The patch provides the same information than sysfs except the
transitions are dynamically allocated, thus they won't show more
events than the ones which actually occurred. There is no longer a
size limitation and it opens the field for more debugging information
where the debugfs is designed for, not sysfs.

The thermal debugfs directory structure tries to stay consistent with
the sysfs one but in a very simplified way:

thermal/
 -- cooling_devices
    |-- 0
    |   |-- clear
    |   |-- time_in_state_ms
    |   |-- total_trans
    |   `-- trans_table
    |-- 1
    |   |-- clear
    |   |-- time_in_state_ms
    |   |-- total_trans
    |   `-- trans_table
    |-- 2
    |   |-- clear
    |   |-- time_in_state_ms
    |   |-- total_trans
    |   `-- trans_table
    |-- 3
    |   |-- clear
    |   |-- time_in_state_ms
    |   |-- total_trans
    |   `-- trans_table
    `-- 4
        |-- clear
        |-- time_in_state_ms
        |-- total_trans
        `-- trans_table

The content of the files in the cooling devices directory is the same
as the sysfs one except for the trans_table which has the following
format:

Transition	Hits
1->0      	246
0->1      	246
2->1      	632
1->2      	632
3->2      	98
2->3      	98

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
[ rjw: White space fixups, rebase ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>