platform/x86: x86-android-tablets: Use platform-device as gpio-keys parent

Use the new x86-android-tablets platform-device as gpio-keys parent
to make it clear that this gpio-keys device was instantiated by
the x86-android-tablets driver.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20230909141816.58358-8-hdegoede@redhat.com