gpio: tps68470: Allow building as module

The gpio-tps68470 driver binds to a tps68470-gpio platform-device which
itself gets instantiated by a special MFD driver from
drivers/platform/x86/intel/int3472/tps68470.c

This MFD driver itself can be built as a module, so it makes no sense to
force the gpio-tps68470 driver to always be built-in.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>