kunit: provide kunit_platform_device_unregister()

kunit: provide kunit_platform_device_unregister()

Tests may want to unregister a platform device as part of the test case
logic. Using the regular platform_device_register() with kunit
assertions may result in a platform device leak or otherwise requires
cumbersome error handling. Provide a function that unregisters a
kunit-managed platform device and drops the release action from the
test's list.

Reviewed-by: David Gow <david@davidgow.net>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/20260522-gpiolib-kunit-v3-2-b15fe6987430@oss.qualcomm.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>