gpio: mockup: Convert to use software nodes

The gpio-mockup driver creates the properties that are shared between
platform and GPIO devices. Because of that, the properties may not
be removed at the proper point of time without provoking a use-after-free
as shown in the following backtrace:

  refcount_t: underflow; use-after-free.
  WARNING: CPU: 0 PID: 103 at lib/refcount.c:28 refcount_warn_saturate+0xd1/0x120
  ...
  Call Trace:
  kobject_put+0xdc/0xf0
  software_node_notify_remove+0xa8/0xc0
  device_del+0x15a/0x3e0

That's why the driver has to manage the lifetime of the software nodes
by itself.

The problem originates from the old device_add_properties() API, but
has been only revealed after the commit bd1e336a ("driver core: platform:
Remove platform_device_add_properties()"). Hence, it's used as a landmark
for backporting.

Fixes: bd1e336a

 ("driver core: platform: Remove platform_device_add_properties()")
Reported-by: Kent Gibson <warthog618@gmail.com>
Tested-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[Bartosz: tweaked local variable placement]
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>