PM / Domains: add debugfs listing of struct generic_pm_domain-s

Add /sys/kernel/debug/pm_genpd/pm_genpd_summary file, which
lists power domains in the system, their statuses and attached devices,
resembling /sys/kernel/debug/clk/clk_summary.

Currently it is impossible to inspect (from userland) whether
a power domain is on or off. And, if it is on, which device blocks it
from powering down. This change allows developers working on
embedded devices power efficiency to list all necessary information
about generic power domains in one place.

The content of pm_genpd/pm_genpd_summary file is generated by iterating
over all generic power domain in the system, and, for each,
over registered devices and over the subdomains, if present.

Example output:
$ cat  /sys/kernel/debug/pm_genpd/pm_genpd_summary
    domain                      status         slaves
           /device                                      runtime status
----------------------------------------------------------------------
a4su                            off
a3sg                            off
a3sm                            on
a3sp                            on
    /devices/e6600000.pwm                               suspended
    /devices/e6c50000.serial                            active
    /devices/e6850000.sd                                suspended
    /devices/e6bd0000.mmc                               active
a4s                             on               a3sp, a3sm, a3sg
    /devices/e6900000.irqpin                            unsupported
    /devices/e6900004.irqpin                            unsupported
    /devices/e6900008.irqpin                            unsupported
    /devices/e690000c.irqpin                            unsupported
    /devices/e9a00000.ethernet                          active
a3rv                            off
a4r                             off              a3rv
    /devices/fff20000.i2c                               suspended
a4lc                            off
c5                              on               a4lc, a4r, a4s, a4su
    /devices/e6050000.pfc                               unsupported
    /devices/e6138000.timer                             active

To enable this feature, compile the kernel with debugfs
and CONFIG_PM_ADVANCED_DEBUG enabled.

Signed-off-by: Maciej Matraszek <m.matraszek@samsung.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>